THE

RAY SOCIETY.

INSTITUTED MDCCCXLIV.

LONDON.

MDCCCLI.

A MONOGRAPH

ON THE SUB-CLASS

CIRRIPEDIA,

WITH

FIGURES OF ALL THE SPECIES.

BY

CHARLES DARWIN, F.R.S., F.G.S.

THE LEPADIDÆ;

OR,

PEDUNCULATED CIRRIPEDES.

LONDON:

PRINTED FOR THE RAY SOCIETY.

MDCCCLI.

C. AND J. ADLARD, PRINTERS, BARTHOLOMEW

PREFACE.

My duty, in acknowledging the great obligations under which I lie to
many naturalists, affords me most sincere pleasure. I had originally
intended to have described only a single abnormal Cirripede, from the
shores of South America, and was led, for the sake of comparison, to
examine the internal parts of as many genera as I could procure. Under
these circumstances, Mr. J. E. Gray, in the most disinterested manner,
suggested to me making a Monograph on the entire class, although he
himself had already collected materials for this same object.
Furthermore, Mr. Gray most kindly gave me his strong support, when I
applied to the Trustees of the British Museum for the use of the public
collection; and I here most respectfully beg to offer my grateful
acknowledgments to the Trustees, for their most liberal and unfettered
permission of examining, and when necessary, disarticulating the
specimens in the magnificent collection of Cirripedes, commenced by Dr.
Leach, and steadily added to, during many years, by Mr. Gray.
Considering the difficulty in determining the species in this class, had
it not been for this most liberal permission by the Trustees, the public
collection would have been of no use to me, or to any other naturalist,
in systematically classifying the Cirripedes.

Previously to Mr. Gray suggesting to me the present Monograph, Mr.
Stutchbury, of Bristol, had offered to intrust to me his truly beautiful
collection, the fruit of many years’ labour. At that time I refused this
most generous offer, intending to confine myself to anatomical
observations; but I have since accepted it, and still have the entire
splendid collection for my free use. Mr. Stutchbury, with unwearied
kindness, further supplied me with fresh specimens for dissection, and
with much valuable information. At about the same period, Mr. Cuming
strongly urged me to take up the subject, and his advice had more weight
with me than that of almost any other person. He placed his whole
magnificent collection at my disposal, and urged me to treat it as if it
were my own: whenever I told him that I thought it necessary, he
permitted me to open unique specimens of great value, and dissect the
included animal. I shall always feel deeply honoured by the confidence
reposed in me by Mr. Cuming and Mr. Stutchbury.

I lie under obligations to so many naturalists, that I am, in truth, at
a loss how to express my gratitude. Mr. Peach, over and over again, sent
me fresh specimens of several species, and more especially of
_Scalpellum vulgare_, which were of invaluable assistance to me in
making out the singular sexual relations in that species. Mr. Peach,
furthermore, made for me observations on several living individuals. Mr.
W. Thompson, the distinguished Natural Historian of Ireland, has sent me
the finest collection of British species, and their varieties, which I
have seen, together with many very valuable MS. observations, and the
results of experiments. Prof. Owen procured for me the loan of some very
interesting specimens in the College of Surgeons, and has always given
me his invaluable advice and opinion, when consulted by me. Professor E.
Forbes has been, as usual, most kind in obtaining for me specimens and
information of all kinds. To the Rev. R. T. Lowe I am indebted for his
particularly interesting collection of Cirripedes from the Island of
Madeira--a collection offering a singular proof what treasures skill and
industry can discover in the most confined locality. The well-known
conchologist, Mr. J. G. Jeffreys, has sent for my examination a very
fine collection of British specimens, together with a copious MS. list
of synonyms, with the authorities quoted. To the kindness of Messrs. M^c
Andrew, Lovell Reeve, G. Busk, G. B. Sowerby, Sen., D. Sharpe,
Bowerbank, Hancock, Adam White, Dr. Baird, Sir John Richardson, and
several other gentlemen, I am greatly indebted for specimens and
information: to Mr. Hancock I am further indebted for several long and
interesting letters on the burrowing of Cirripedes.

Nor are my obligations confined to British naturalists. Dr. Aug. Gould,
of Boston, has most kindly transmitted to me some very interesting
specimens; as has Prof. Agassiz other specimens collected by himself in
the Southern States. To Mr. J. D. Dana, I am much indebted for several
long letters, containing original and valuable information on points
connected with the anatomy of the Cirripedia. To Mr. Conrad I am
likewise indebted for information and assistance. Both the celebrated
Professors, Milne Edwards and Müller, have lent me, from the great
public collections under their charge, specimens which I should not
otherwise have seen. To Professor W. Dunker, of Cassel, I am indebted
for the examination of his whole collection. I have, in a former
publication, expressed my thanks to Professor Steenstrup, but I must be
permitted here to repeat them, for a truly valuable present of a
specimen of the _Anelasma squalicola_ of this work. I will conclude my
thanks to all the above British and foreign naturalists, by stating my
firm conviction, that if a person wants to ascertain how much true
kindness exists amongst the disciples of Natural History, he should
undertake, as I have done, a Monograph on some tribe of animals, and let
his wish for assistance be generally known.

Had it not been for the Ray Society, I know not how the present volume
could have been published; and therefore I beg to return my most sincere
thanks to the Council of this distinguished Institution. To Mr. G. B.
Sowerby, Junr., I am under obligations for the great care he has taken
in making preparatory drawings, and in subsequently engraving them. I
believe naturalists will find that the ten plates here given are
faithful delineations of nature.

In Monographs, it is the usual and excellent custom to give a history of
the subject, but this has been so fully done by Burmeister, in his
‘Beiträge zur Naturgeschichte der Rankenfüsser,’ and by M. G. Martin St.
Ange, in his ‘Mémoire sur l’Organisation des Cirripèdes,’ that it would
be superfluous here to repeat the same list of authors. I will only add,
that since the date, 1834, of the above works, the only important papers
with which I am acquainted, are, 1st. Dr. Coldstream ‘On the Structure
of the Shell in Sessile Cirripedes,’ in the ‘Encyclopædia of Anatomy and
Physiology;’ 2d. Dr. Lovén ‘On the Alepas squalicola,’ (‘Ofversigt of
Kongl. Vetens.,’ &c. Stockholm, 1844, p. 192,) giving a short but
excellent account of this abnormal Cirripede; 3d. Professor Leidy’s very
interesting discovery, (‘Proceedings of the Academy of Natural
Sciences,’ Philadelphia, vol. iv, No. I, Jan. 1848,) of eyes in a mature
Balanus; 4th. Mr. A. Hancock’s Memoir, (‘Annals of Natural History, 2d
series, Nov. 1849,) on his Alcippe lampas, the type of a new order of
Cirripedes; 5th. Mr. Goodsir’s Paper, (‘Edinburgh New Philosoph.
Journal,’ July 1843,) on the Larvæ in the First Stage of Development in
Balanus; 6th. Mr. C. Spence Bate’s valuable Paper on the same subject,
lately published, (Oct. 1851,) in the ‘Annals of Natural History;’ and
lastly, M. Reinhardt has described, in the ‘Copenhagen Journal of
Natural History, Jan. 1851,’ the _Lithotrya Nicobarica_, and has
discussed its powers of burrowing into rocks.

I have given the specific or diagnostic characters, deduced from the
external parts alone, in both Latin and English. As I found, during the
progress of this work, that a similarly abbreviated character of the
softer internal parts, was very useful in discriminating the species, I
have inserted it after the ordinary specific character.

In those cases in which a genus includes only a single species, I have
followed the practice of some botanists, and given only the generic
character, believing it to be impossible, before a second species is
discovered, to know which characters will prove of specific, in
contradistinction to generic, value.

In accordance with the Rules of the British Association, I have
faithfully endeavoured to give to each species the first name attached
to it, subsequently to the introduction of the binomial system, in 1758,
in the tenth edition[1] of the ‘Systema Naturæ.’ In accordance with the
Rules, I have rejected all names before this date, and all MS. names. In
one single instance, for reasons fully assigned in the proper place, I
have broken through the great law of priority. I have given much fewer
synonyms than is usual in conchological works; this partly arises from
my conviction that giving references to works, in which there is not any
original matter, or in which the Plates are not of a high order of
excellence, is absolutely injurious to the progress of natural history,
and partly, from the impossibility of feeling certain to which species
the short descriptions given in most works are applicable;--thus, to
take the commonest species, the _Lepas anatifera_, I have not found a
single description (with the exception of the anatomical description by
M. Martin St. Ange) by which this species can be certainly discriminated
from the almost equally common _Lepas Hillii_. I have, however, been
fortunate in having been permitted to examine a considerable number of
authentically named specimens, (to which I have attached the sign (!)
used by botanists,) so that several of my synonyms are certainly
correct.

   [1] In the Rules published by the British Association, the 12th
   edition, (1766,) is specified, but I am informed by Mr.
   Strickland that this is an error, and that the binomial method
   was followed in the 10th edition.

The Lepadidæ, or pedunculated Cirripedes, have been neglected under a
systematic point of view, to a degree which I cannot quite understand:
no doubt they are subject to considerable variation, and as long as the
internal surfaces of the valves and all the organs of the animal’s body,
are passed over as unimportant, there will occasionally be some
difficulty in the identification of the several forms, and still more in
settling the limits of the variability of the species. But I suspect the
pedunculated Cirripedes have, in fact, been neglected owing to their
close affinity, and the consequent necessity of their being included in
the same Work with the Sessile Cirripedes; for these latter will ever
present, I am fully convinced, insuperable difficulties in their
identification by external characters alone.

I will here only further remark, that in the Introduction I have given
my reasons for assigning distinct names to the several Valves, and to
some parts of the included animal’s body; and that in the Introductory
Remarks, under the general description of the Lepadidæ, I have given an
abstract of my Anatomical Observations.

CORRIGENDA AND ADDENDA.

Page

12, twenty lines from bottom, _for_ “hinder pair of true thoracic
          limbs,” _read_ “pair of true thoracic limbs.”

42, 43. I should have added, that the number of the segments in the
          cirri increases with the age of the specimen; but that the
          relative numbers in the different cirri keep, as far as I have
          seen, nearly constant; hence the numbers are often given in
          the descriptions.

99 et passim, _for_ Pæcilasma, _read_ Poecilasma.

156. In a foot-note, I have alluded to a new genus of sessile
          Cirripedes, under the name of Siphonicella, I now find that
          this species has been called, by Professor Steenstrup,
          _Xenobalanus globicipitis_.

MONOGRAPH

ON

THE CIRRIPEDIA.

INTRODUCTION.

I should have been enabled to have made this Volume more complete, had I
deferred its publication until I had finished my examination of all the
other known Cirripedes; but my work would thus have been rendered
inconveniently large. Until this examination is completed, it will be
more prudent not to discuss, in detail, the position of the Lepadidæ
amongst the Cirripedia, or of these latter in the great class of
Crustacea, to which they now, by almost universal consent, have been
assigned. I may, however, remark that I believe the Cirripedia do not
approach, by a single character, any animal beyond the confines of the
Crustacea: where such an approach has been imagined, it has been founded
on erroneous observations; for instance, the closed tube within the
stomach, described by M. Martin St. Ange (to whose excellent paper I am
greatly indebted), as indicating an affinity to the Annelides, is, I am
convinced, nothing but a strong epithelial lining, which I have often
seen ejected with the excrement. Again, a most distinguished author has
stated that the Cirripedia differ from the Crustacea:--1st. In having “a
calcareous shell and true mantle;” but there is no essential difference,
as shown by Burmeister, in the shells in these two classes; and
Cirripedes certainly have no more claim to a mantle than have the
bivalve entomostraca. 2d. “In the sexes joined in one individual;” but
this, as we shall see, is not constant, nor of very much weight, even if
constant. 3d. “In the body not being ringed;” but if the outer
integument of the thorax of any Cirripede be well cleaned, it will be
seen, (as was long ago shown by Martin St. Ange), to be most distinctly
articulated. 4th. “In having salivary glands;” but these glands are, in
truth, the ovaria. 5th. “In the liver being formed on the molluscous
type;” I do not think this is the case, but I do not quite understand
the point in question. 6th. “In not having a head or organs of sense;”
this is singularly erroneous: Professor Leidy has shown the existence of
eyes in the mature Cirripede; the antennæ, though preserved, certainly
become functionless soon after the last metamorphosis; but there exist
other organs of sense, which I believe serve for smelling and hearing:
and lastly, so far from there being no head, the whole of the Cirripede
externally visible, consists exclusively of the three anterior segments
of the head.

The sub-class, Cirripedia, can be divided into three Orders; the first
of which, mainly characterised by having six pair of thoracic cirri,
includes all common Cirripedes: these latter may be divided into three
families,--the Lepadidæ, or pedunculated Cirripedes, the subject of the
present memoir; the Verrucidæ containing the single genus Verruca or
Clisia; and, lastly, the Balanidæ, which consist of two very distinct
sub-families, the Balaninæ and Chthamalinæ. Of the other two Orders
above alluded to, one will, I believe, contain the remarkable burrowing
genus Alcippe, lately described by Mr. Hancock, and a second burrowing
genus, or rather family, obtained by me on the coast of South America.
The third Order is highly singular, and differs as much from all other
Cirripedes as does a Lernæa from other crustaceans; it has a suctorial
mouth, but is destitute of an anus; it has not any limbs, and is as
plainly articulated as the larva of a fly; it is entirely naked, without
valves, carapace, or capitulum, and is attached to the Cirripede, in the
sack of which it is parasitic, by _two_ distinct threads, terminating in
the usual larval, prehensile antennæ. I intend to call this Cirripede,
Proteolepas. I mention it here for the sake of calling attention to any
parasite at all answering to this description.

NOMENCLATURE OF THE VALVES.

[Illustration: Figure I.

CAPITULUM.]

[Illustration: Figure II.

SCUTUM of LEPAS.]

[Illustration: Figure III.

TERGUM of LEPAS.]

Although the present volume is strictly systematic, I will, under the
general description of the Lepadidæ, give a very brief abstract of some
of the most interesting points in their internal anatomy, and in the
metamorphoses of the whole class, which I hope hereafter to treat, with
the necessary illustrations, in detail. I enter on the subject of the
metamorphoses the more readily, as by this means alone can the
homologies of the different parts be clearly understood.

_On the Names given to the different parts of Cirripedes._

I have unwillingly found it indispensable to give names to several
valves, and to some few of the softer parts of Cirripedes. The
accompanying figure of an imaginary Scalpellum includes every valve; the
two most important valves of Lepas are also given, in which the
direction of the lines of growth and general shape differ from those of
Scalpellum as much as they do in any genus. The names which I have
imposed will, I hope, be thus acquired without much difficulty.

Whoever will refer to the published descriptions of recent and fossil
Cirripedia, will find the utmost confusion in the existing nomenclature:
thus, the valve named in the woodcut the _Scutum_, has been designated
by various well-known naturalists as the “ventral,” the “anterior,” the
“inferior,” the “ante-lateral,” and the “latero-inferior” valve; the
first two of these titles have, moreover, been applied to the rostrum or
rostral valve of sessile Cirripedes. The _Tergum_ has been called the
“dorsal,” the “posterior,” the “superior,” the “central,” the
“terminal,” the “postero-lateral,” and the “latero-superior” valve. The
_Carina_ has received the first two of these identical epithets, viz.
the “dorsal” and the “posterior;” and likewise has been called the
“keel-valve.” The confusion, however, becomes far worse, when any
individual valve is described, for the very same margin which is
anterior or inferior in the eyes of one author, is the posterior or
superior in those of another; it has often happened to me that I have
been quite unable even to conjecture to which margin or part of a valve
an author was referring. Moreover, the length of these double titles is
inconvenient. Hence, as I have to describe all the recent and fossil
species, I trust I may be thought justified in giving short names to
each of the more important valves, these being common to the
pedunculated and sessile Cirripedes.

The part supported by the peduncle, and which is generally, though not
always, protected by valves, I have designated the _Capitulum_.

The title of _Peduncle_, which is either naked or squamiferous, requires
no explanation; the scales on it, and the lower valves of the capitulum,
are arranged in whorls, which, in the Latin specific descriptions, I
have called by the botanical term of verticillus.

I have applied the term _Scutum_ to the most important and persistent of
the valves, and which can generally be recognised by the hollow giving
attachment to the adductor scutorum muscle, from the resemblance which
the two valves taken together bear to a shield, and from their office of
protecting the front side of the body. From the protection afforded by
the two _Terga_ to the dorso-lateral surface of the animal, these valves
have been thus called. The term _Carina_[2] is a mere translation of
the name already used by some authors, of Keel-Valve.

   [2] In the Carina of Fossil Species of Scalpellum, I have found
   it necessary to distinguish different parts, viz., A, the tectum,
   of which half is seen; B, the parietes; and C, the
   intra-parietes.

The _Rostrum_ has been so called from its relative position to the
carina or keel. There is often a _Sub-carina_ and a _Sub-rostrum_.

The remaining valves, when present, have been called _Latera_; there is
always one large upper one inserted between the lower halves of the
scuta and terga, and this I have named the Upper Latus or Latera; the
other latera in Pollicipes are numerous, and require no special names;
in Scalpellum, where there are at most only three pair beneath the Upper
Latera, it is convenient to speak of them (_vide_ Woodcut, I,) as the
_Carinal_, _Infra-median_, and _Rostral Latera_.

As each valve often requires (especially amongst the fossil species) a
distinct description, I have found it indispensable to give names to
each margin. These have mostly been taken from the name of the adjoining
valve, (see fig. I.) In Lepas, Pollicipes, &c., the margin of the scutum
adjoining the tergum and upper latus, is not divided (fig. II) into two
distinct lines, as it is in Scalpellum, and is therefore called the
Tergo-lateral margin. In Scalpellum (fig. I) these two margins are
separately named Tergal and Lateral. The angle formed by the meeting of
the basal and lateral or tergo-lateral margins, I call the Baso-lateral
angle; that formed by the basal and occludent margins, I call, from its
closeness to the Rostrum, the Rostral angle. In Pollicipes the carinal
margin of the tergum can be divided into an upper and lower carinal
margin; of this there is only a trace (fig. I) in Scalpellum.

That margin in the scuta and terga which opens and _shuts_ for the
exsertion and retraction of the cirri, I have called the Occludent
margin. In the terga of Lepas (fig. III) and some other genera, the
occludent margin is highly protuberant and arched, or even formed of two
distinct sides.

Occasionally, I have referred to what I have called the _primordial
valves_: these are not calcified; they are formed at the first
exuviation, when the larval integuments are shed: in mature Cirripedes
they are always seated, when not worn away, on the umbones of the
valves.

The membrane connecting the valves, and forming the peduncle, and
sometimes in a harder condition replacing the valves, I have often found
it convenient to designate by its proper chemical name of _Chitine_,
instead of by horny, or other such equivalents. When this membrane at
any articulation sends in rigid projections or crests, for the
attachment of muscles or any other purpose, I call them, after Audouin,
_apodemes_. For the underlying true skin, I use the term _corium_.

The animal’s body is included within the capitulum, within what I call
the _sack_ (see Pl. IV, figs. 2 and 8´ _a_, and Pl. IX, fig. 4). The
body consists of the _thorax_ supporting the cirri, and of an especial
enlargement, or downward prolongation of the thorax, which includes the
stomach, and which I have called the _prosoma_. (Pl. IX, fig. 4 _n_).
The cirri are composed of two arms or _rami_, supported on a common
segment or support, which I call the _pedicel_. The _caudal appendages_
are two little projections, either uni-or multi-articulate (Pl. IV, fig.
8´ _a_), on each side of the anus, and just above the long
proboscis-like penis. On the thorax and prosoma, or on the pedicels of
the cirri, there are in several genera, long, thin, tapering filaments,
which have generally been supposed to serve as branchiæ; these I call
simply _filaments_, or _filamentary appendages_ (Pl. IX, fig. 4 _g-l_).
The mouth (fig. 4 _b_) is prominent, and consists of _palpi_ soldered to
the _labrum_; _mandibles_, _maxillæ_, and _outer maxillæ_, these latter
serve as an under lip; to these several organs I sometimes apply the
title used by Entomologists, of “trophi.” Beneath the outer maxillæ,
there are either two simple orifices or tubular projections; these, I
believe, serve as organs of smell, and have hence called them the
_olfactory orifices_. Within the sack, there are often two sheets of ova
(Pl. IV, fig. 2 _b_), these I call (after Steenstrup, and other
authors) the _ovigerous Lamellæ_; they are united to two little folds of
skin (Pl. IV, fig. 2 _f_), which I call the _ovigerous Fræna_.

From the peculiar curved position which the animal’s body occupies
within the capitulum, I have found it far more convenient (not to
mention the confusion of nomenclature already existing) to apply the
term Rostral instead of ventral, and Carinal instead of dorsal, to
almost all the external and internal parts of the animal. Cirripedes
have generally been figured with their surfaces of attachment downwards,
hence I speak of the lower or Basal margins and angles, and of those
pointing in an opposite direction as the Upper; strictly speaking, as we
shall presently see, the exact centre of the usually broad and flat
surface of attachment is the anterior end of the animal, and the upper
tips of the Terga, the posterior end of that part of the animal which is
externally visible; but in some cases, for instance in Coronula, where
the base is _deeply concave_, and where the width of the shell far
exceeds the depth, it seemed almost ridiculous to call this, the
anterior extremity; as likewise does it in Balanus to call the united
tips of the Terga, lying deeply within the shell, the most posterior
point of the animal, as seen externally.

I have followed the example of Botanists, and added the interjection [!]
to synonyms, when I have seen an authentic specimen bearing the name in
question.

Every locality, under each species, is given from specimens ticketed in
a manner and under circumstances appearing to me worthy of full
confidence,--the specific determination being in each case made by
myself.

CLASS--CRUSTACEA. SUB-CLASS--CIRRIPEDIA.

FAMILY--LEPADIDÆ.

_Cirripedia pedunculo flexili, musculis instructo: scutis[3] musculo
adductore solummodô instructis: valvis cæteris, siquæ adsunt, in annulum
immobilem haud conjunctis._

Cirripedia having a peduncle, flexible, and provided with muscles.
Scuta[3] furnished only with an adductor muscle: other valves, when
present, not united into an immovable ring.

    Metamorphoses; larva, first stage, pp. 9-12; larva, second
    stage, p. 13; larva, last stage, p. 14; its carapace, ib.;
    acoustic organs, p. 15; antennæ, ib.; eyes, p. 16; mouth, p. 17;
    thorax and limbs, p. 18; abdomen, p. 19; viscera, ib.; immature
    cirripede, p. 20; homologies of parts, p. 25.

    Description of mature Lepadidæ, p. 28; capitulum, ib.; peduncle,
    p. 31; attachment, p. 33; filamentary appendages, p. 38; shape
    of body, and muscular system, p. 39; mouth, ib.; cirri, p. 42;
    caudal appendages, p. 43; alimentary canal, 44; circulatory
    system, p. 46; nervous system, ib.; eyes, p. 49; olfactory
    organs, p. 52; acoustic(?) organs, p. 53; male sexual organs, p.
    55; female organs, p. 56; ovigerous lamellæ, p. 58; ovigerous
    fræna, ib.; exuviation, p. 61; rate of growth, ib.; size, ib.;
    affinities of family, p. 64; range and habitats, p. 65;
    geological history, p. 66.

   [3] The meaning of this and all other terms is given in the
   Introduction, at pp. 3-7.

_Metamorphoses._--I will here briefly describe the Metamorphoses, as far
as known, common to all Cirripedia, but more especially in relation to
the present family. I may premise, that since Vaughan Thompson’s capital
discovery of the larvæ in the last stage of development in Balanus, much
has been done on this subject: this same author subsequently
published[4] in the ‘Philosophical Transactions,’ an account of the
larvæ of Lepas and Conchoderma (Cineras) in the first stage; and seeing
how totally distinct they were from the larva of the latter stage in
Balanus, he erroneously attributed the difference to the difference in
the two families, instead of to the stage of development. Burmeister[5]
first showed, and the discovery is an important one, that in Lepas the
larvæ pass through two totally different stages. This has subsequently
been proved by implication to be the case in Balanus, by Goodsir,[6] who
has given excellent drawings of the larva in the first stage; and quite
lately, Mr. C. Spence Bate, of Swansea, has made other detailed
observations and drawings of the larvæ of five species in this same
early stage, and has most kindly permitted me to quote from his
unpublished paper[7]. I am enabled to confirm and generalise these
observations, in all the Cirripedes in the Order containing the Balanidæ
and Lepadidæ.

   [4] Philosophical Transactions, 1835, p. 355, Pl. vi.

   [5] Beiträge zur Naturgeschichte der Rankenfüsser, 1834. Mr. J.
   E. Gray, however, briefly described, in 1833, (Proceedings,
   Zoological Society, October,) the larva in the first stage of
   Balanus; in this notice the anterior end of the larva is
   described as the posterior.

   [6] Edinburgh New Philosophical Journal, July 1843, Pls. iii and
   iv.

   [7] This will appear in the October number (1851) of the ‘Annals
   of Natural History.’

The ova, and consequently the larvæ of the Lepadidæ, in the _First
Stage_, whilst within the sack of the parent, vary in length from .007
to .009 in Lepas, to .023 of an inch in Scalpellum: my chief examination
of these larvæ has been confined to those of _Scalpellum vulgare_; but I
saw them in all the other genera. The larva is somewhat depressed, but
nearly globular; the carapace anteriorly is truncated, with lateral
horns; the sternal surface is flat and broad, and formed of thinner
membrane than the dorsal. The horns just alluded to are long in Lepas
and short in Scalpellum; their ends are either rounded and excessively
transparent, or, as in Ibla, furnished with an abrupt, minute, sharp
point: within these horns, I distinctly saw a long filiformed organ,
bearing excessively fine hairs in lines, so exactly like the long
plumose spines on the prehensile antennæ of the larvæ in the last stage;
that I have not the least doubt, that these horns are the cases in which
antennæ are in process of formation. Posteriorly to them, on the
sternal surface, near each other, there are two other minute, doubly
curved, pointed horns, about .004 in length, directed posteriorly; and
within these I again saw a most delicate articulated filiformed organ on
a thicker pedicel: in an excellent drawing, by Mr. C. S. Bate, of the
larva of a Chthamalus (_Balanus punctatus_ of British authors), after
having kept alive and moulted once, these organs are distinctly shown as
articulated antennæ (without a case), directed forwards: hence, before
the first moult in Scalpellum, we have two pair of antennæ in process of
formation. Anteriorly to the bases of these smaller antennæ is seated
the heart-shaped eye, (as I believe it to be,) .001 in diameter, with
apparently a single lens, surrounded, except at the apex, by
dark-reddish pigment-cells. In some cases, as in some species of Lepas,
the larvæ, when first excluded from the egg, have not an eye, or a very
imperfect one.

There are three pairs of limbs, seated close together in a longitudinal
line, but some way apart in a transverse direction: the first pair
always consists of a single spinose ramus, it is not articulated in
Scalpellum, but is multi-articulate in some genera; it is directed
forwards. The other two pair have each two rami, supported on a common
haunch or pedicel: in both pair, the longer ramus is multi-articulate,
and the shorter ramus is without articulations, or with only traces of
them: the longer spines borne on these limbs (at least, in Scalpellum
and Chthamalus,) are finely plumose. The abdomen terminates, a little
beyond the posterior end of the carapace, in a slightly upturned horny
point; a short distance anteriorly to this point, a strong, spinose,
forked projection depends from the abdominal surface.

Messrs. V. Thompson, Goodsir, and Bate, have kept alive for several days
the larvæ of Lepas, Conchoderma, Balanus, Verruca, and Chthamalus, and
have described the changes which supervene between the first and third
exuviations. The most conspicuous new character is the great elongation
of the posterior point of the carapace into an almost filiform, spinose
point in Lepas, Conchoderma, Chthamalus, and Balanus, but not according
to Goodsir, in one of the species of the latter genus. The posterior
point, also, of the abdomen becomes developed in Balanus (Goodsir) into
two very long, spear-like processes, serrated on their outer sides; in
Lepas and Conchoderma, according to Thompson, into a single, tapering
spinose projection; and in Chthamalus, as figured by Mr. Bate, the
posterior bifid point, as well as the depending ventral fork, increase
much in size. Another important change, which has been particularly
attended to by Mr. Bate, is the appearance of spinose projections and
spines (some of which are thick, curved, and strongly plumose, or,
almost pectinated along their inner sides) on the pedicels and lower
segments of the shorter rami of the two posterior pairs of limbs.

The mouth in its earliest condition alone remains to be described; in
_S. vulgare_, it is seated on a very slight prominence, in a most
remarkable situation, namely, in a central point between the bases of
the three pairs of legs. I traced by dissection the oesophagus for some
little way, until lost in the cellular and oily matter filling the whole
animal, and it was directed anteriorly, which is the direction that
might have been expected, from the course followed by the oesophagus in
the larva in the last stage, and in mature Cirripedes. Mr. A. Hancock
has called my attention to a probosciformed projection on the under side
of the larva of _Lepas fascicularis_, when just escaped from the egg.
Mr. Bate has described this same proboscis in Balanus and Chthamalus,
and states the important fact, that it is capable of being moved by the
animal; and, lastly, I have seen it in an Australian Chthamalus, and in
Ibla, of remarkable size. This proboscis, which is always directed
posteriorly, (like the mouth in the mature animal,) certainly answers to
the mouth as made out by dissection in Scalpellum; and I believe I saw,
as has Mr. Bate, a terminal orifice: it certainly does not possess any
trophi. In Ibla (in which the larva is large enough for dissection), the
base of the proboscis arises posteriorly to the first pair of legs, and
the orifice at the other end reaches beyond or posteriorly to the point,
where the mouth in Scalpellum opens, namely between the middle pair of
legs. The mouth being either so largely probosciformed or seated only on
a slight eminence, in two genera so closely allied as Ibla and
Scalpellum, and (judging from Mr. Thompson’s figures, and from what I
have seen myself,) in the species of the same genus Lepas, is a singular
difference: in the cases in which, at first, the proboscis is absent, it
would probably soon be developed. I cannot but suppose that the inwardly
directed spines on the bases of the two posterior legs, which are so
rapidly developed, serve some important end, namely, as organs of
prehension for the larvæ, like the mandibles and maxillæ of mature
Cirripedes, for seizing their prey, and conveying it to their moveable
mouths, conveniently seated for this purpose.

The first pair of legs answers, as I believe from reasons hereafter to
be assigned, to the outer pair of maxillipods in the higher crustacea;
and the other four legs to the first two pair of thoracic limbs in these
same crustacea; this being the case, the highly remarkable position of
the mouth in the larva, either between the bases of the two posterior
pair of legs, or at least posteriorly to the first pair, together with
the probable functions of the spiny points springing from the basal
segments of the two hinder pair of true thoracic limbs, forcibly bring
to mind the anomalous structure of the mouth being situated in the
middle of the under side of the thorax, in Limulus,--that most ancient
of crustaceans, and therefore one likely to exhibit a structure now
embryonic in other orders. I will only further remark, that I suspect
that the truncation of the anterior end of the carapace, has been
effected by the segments having been driven inwards, and consequently,
that the larger antennæ within the lateral horns, though standing more
in front than the little approximate pair, are normally the posterior
of the two pair. According to Milne Edwards, the posterior pair are
normally seated outside the anterior pair, and this is the case with
those within the lateral horns.

_Larva in the Second Stage._--Notwithstanding the considerable changes,
already briefly given, which the larva undergoes during the first two or
three exuviations after leaving the egg, all these forms may be
conveniently classed under the first stage. The larva in the Second
stage is known only from a single specimen described, figured, and found
by Burmeister,[8] adhering to sea-weed in the midst of other larvæ of
Lepas in the last stage. In its general shape and compressed form, it
seems to come nearer to the last than to the first stage. It has only
three pair of legs, situated much more posteriorly on the body than in
the first stage, and all directed posteriorly; they are much shorter
than heretofore, and resemble rather closely those of the last stage,
with the important exception that the first pair has only one ramus. It
is this circumstance which leaves no doubt on my mind, that we here have
the three pair of limbs, of the first stage, metamorphosed. The body is
prolonged some way behind these limbs, and ends in a blunt, rounded
point, in which, probably, are developed the three posterior pair of
legs and the abdomen of the larva in the last stage. The mouth is now
seated some way anteriorly to the limbs, is large and probosciformed,
and is, I presume, still destitute of trophi. There are now two closely
approximate eyes, but as yet both are _simple_. The smaller pair of
antennæ has disappeared. The whole animal was attached to the sea-weed
by a (I presume, pair of,) “fleischigen Fortsatz,” which Burmeister
considers as the prehensile antennæ, to be presently described, in an
early state of development. I have little doubt that this is correct,
for in an abnormal Cirripede of another order, in which the larva
appears in the _first_ stage with prehensile antennæ, the eggs have two
great projecting horns including these organs, and attached by their
tips, through some unknown means, to the sack of the parent, apparently
in the same manner as Burmeister’s larva was attached to the sea-weed. I
will only further remark on the larva of this Second stage, that its
chief development since the first stage, has been towards its anterior
end. The next great development, to be immediately described, is towards
the posterior end of the animal.

   [8] Beiträge zur Naturgeschichte der Rankenfüsser, s. 16, Tab. i,
   figs. 3, 4.

_Larva, Last Stage._--My chief examination has been directed, at this
stage of development, to the larvæ of _Lepas australis_, which are of
unusual size, namely, from .065 to even almost .1 of an inch in length;
I examined, however, the larvæ of several other species of Lepas, of
Ibla and of Balanus, with less care, but sufficiently to show that in
all essential points of organisation they were identical; this, indeed,
might have been inferred from the similarity of the larval prehensile
antennæ, preserved in the bases of all mature Cirripedes, and which I
have carefully inspected in almost every genus. The larvæ in this final
stage, in most of the genera, have increased many times in size since
their exclusion from the egg; for instance, in _Lepas australis_, from
.007 to .065, or even to .1 of an inch. They are now much compressed,
nearly of the shape of a cypris or mussel-shell, with the anterior end
the thickest, the sternal surface nearly or quite straight, and the
dorsal arched. Almost the whole of what is externally visible consists
of the carapace; for the thorax and limbs are hidden and enclosed by its
backward prolongation; and even at the anterior end of the animal, the
narrow sternal surface can be drawn up, so as to be likewise enclosed.
As in several Stomapod crustaceans, the part of the head bearing the
antennæ and organs of sense, in front of the mouth, equals, or even
exceeds in length, and more than exceeds in bulk, the posterior part of
the body, consisting of the enclosed thorax and abdomen. I will now
briefly describe, in the following order, the carapace, the organs of
sense, mouth, thorax and limbs, abdomen, and internal viscera.

The form of the _Carapace_ has been sufficiently described; it consists
of thick chitine membrane, marked with lines, and sometimes with stars
and other patterns; it is obscurely divided into two halves by a line or
suture along part of the dorsal margin; these halves or two valves are
drawn together by an adductor muscle, in the same relative position as
in the mature Cirripede. The part overhanging and enclosing the thorax
is lined by an excessively delicate membrane, obviously homologous with
the lining of the sack in the mature animal, and is nothing but a
duplicature of the carapace, rendered very thin from being on the under
or protected side: a layer of true skin or corium, probably double,
separates these two folds.

_Acoustic Organs._--On the borders of the carapace, at the anterior end,
on the sternal surface, there are two minute orifices, in _L. australis_
.002 in diameter, sometimes having a distinct border round them; the
membrane of the carapace on the inside is prolonged upwards and inwards
in two short funnel-shaped tubes, lodged in closed sacks of the corium:
within these sacks on each side a delicate bag is suspended, and hangs
in the mouth of the above funnel; at the upper end a large nerve could
be distinctly seen to enter the bag: I cannot doubt that this is a
sense-organ; from its position and from the animal not feeding (as we
shall presently see), I conclude that it is an acoustic organ.

_Antennæ._--These are large and conspicuous; they are attached very
obliquely on the sternal surface, a little way from the anterior end of
the carapace, beyond which, when exserted, they extend;[9] they can (at
least in Ibla) be retracted within the carapace. They consist of three
segments: the first or basal one is much larger than the others, and
apparently always has a single spine on the outer distal margin. The
second segment consists either of a large, thin, circular, sucking disc,
or is hoof-like (Tab. V, figs. 5, 10, 11, 12); in all cases it is
furnished with one or more spines, (seven very long ones in Lepas,) on
the exterior-hinder margin. The third and ultimate segment is small; it
is articulated on the upper surface of the disc, and is directed
rectangularly outwards; it is sometimes notched, and even shows traces
of being bifid; it bears about seven spines at the end; some of these
spines are hooked, others simple, and in _Lepas_ and _Conchoderma_, two
or three are very long, highly flexible, and plumose, a double row of
excessively fine hairs being articulated on them. I can hardly doubt
that these latter spines, (within which the purple corium could be seen
to enter a little way,) floating laterally outwards, serve as feelers.
The antennæ, at first, are well furnished with muscles. They serve, in
Lepas, according to Mr. King, and in Balanus, according to Mr. Bate, and
as I saw myself in another unnamed order, for the purpose of walking,
one limb being stretched out before the other; but their main function
is to attach the larva for its final metamorphosis into a Cirripede. The
disc can adhere even to so smooth a surface as a glass tumbler.[10] The
attachment is at first manifestly voluntary, but soon becomes
involuntary and permanent, being effected by special and most remarkable
means, which will be most conveniently described in a later part of this
Introduction. I will here only state that I traced with ease the two
cement-ducts running from two large glandular bodies, to within the
antennæ up to the discs.

   [9] Mr. J. D. Dana, who has examined these organs in the larvæ of
   Lepas, informs me in a letter, that in his opinion they
   “correspond with the inferior antennæ, the superior being
   wanting, as in most Daphnidæ.” He continues--“I know of no case
   in which the inferior are obsolete when the superior are
   developed; but the reverse is often true.” In position these
   antennæ certainly correspond to the inferior and central pair of
   the larva in the first stage, which belong, as it would appear,
   to the first segment of the body; but judging from the drawing by
   Burmeister of the larva in the second stage, I am, in some
   respects, more inclined to consider that they correspond to the
   larger pair seen within the lateral horns of the carapace in the
   first stage.

   [10] Rev. B. L. King. Annual Report of B. Institution of
   Cornwall, 1848, p. 55.

_Eyes._--Close behind the basal articulations of the antennæ, the
sternal surface consists of two approximate, elongated, narrow, flat
pieces, or segments. These Burmeister considers as the basal segments
of the antennæ: as they are not cylindrical, I do not see the grounds
for this conclusion: their posterior ends are rounded, and the membrane
forming them is reflected inwards, in the form of two, forked, horny
apodemes, together resembling two letters, =UU=, close together; these
project up, inside the animal, for at least one third of its thickness
from the sternal to the dorsal surface. The two great, almost spherical
eyes in _L. australis_, each 1/150th of an inch in diameter, are
attached to the outer arms, thus, =°UU°=, in the position of the two
full stops. Hence the eyes are included within the carapace. Each eye
consists of eight or ten lenses, varying in diameter in the same
individual from 1/2000 to 3/2000th of an inch, enclosed in a common
membranous bag or cornea, and thus attached to the outer apodemes. The
lenses are surrounded half way up by a layer of dark pigment-cells. The
nerve does not enter the bluntly-pointed basal end of the common eye,
but on one side of the apodeme. The structure here described is exactly
that found, according to Milne Edwards, in certain crustacea. In
specimens _just attached_, in which no absorption has taken place, two
long muscles with transverse striæ may be found attached to the knobbed
tips of the two middle arms of the two =°UU°=, and running up to the
antero-dorsal surface of the carapace, where they are attached; other
muscles (without transverse striæ) are attached round the bases, on both
sides of both forks. The action of these muscles would inevitably move
the eyes, but I suspect that their function may be to draw up the
narrow, deeply folded, sternal surface, and thus cause the retraction of
the great prehensile antennæ within the carapace.

_Mouth._--This is seated in exactly the same position as in the mature
Cirripede, on a slight prominence, fronting the thoracic limbs, and so
far within the carapace, that it was obviously quite unfitted for the
seizure of prey; and it was equally obvious, that the limbs were
natatory, and incapable of carrying food to the mouth. This enigma was
at once explained by an examination of the mouth, which was found to be
in a rudimentary condition and absolutely closed, so that there would be
no use in prey being seized. Underneath this slightly prominent and
closed mouth, I found all the masticatory organs of a Cirripede, in an
immature condition. The state of the mouth will be at once understood,
if we suppose very fluid matter to be poured over the protuberant mouth
of a Cirripede, so as to run a little way down, in the shape of internal
crests, between the different parts, and in the shape of a short,
shrivelled, certainly closed tube, a little way (.008 of an inch in _L.
australis_) down the oesophagus. Hence, the larva in this, its last
stage, cannot eat; it may be called a _locomotive Pupa_;[11] its whole
organisation is apparently adapted for the one great end of finding a
proper site for its attachment and final metamorphosis.

   [11] M. Dujardin has lately (‘Comptes Rendus,’ Feb. 5, 1850, as
   cited in ‘Annals of Nat. History,’ vol. v, p. 318,) discovered
   that the “Hypopi are Acari with eight feet, without either mouth
   or intestine, and which, being deprived of all means of
   alimentation, fix themselves at will, so as to undergo a final
   metamorphosis, and they become Gamasi or Uropodi.” Here, then, we
   have an almost exactly analogous case. M. Dujardin asks--“Ought,
   therefore, the Hypopi to be called larvæ, when, under that
   denomination, have hitherto been comprised animals capable of
   nourishing themselves?”

_Thorax and Limbs._--The thorax is much compressed, and consists of six
segments, corresponding with the six pair of natatory legs; the anterior
segments are much plainer (even the first being distinctly separated by
a fold from the mouth), than the posterior segments, which is exactly
the reverse of what takes place in the mature Cirripede; in the latter,
the first segment is confounded with the part bearing the mouth. The
epimeral elements of the thorax are distinguishable; the sternal surface
is very narrow, and is covered with complicated folds and ridges. The
six pair of legs are all close, one behind the other, and all are alike
in having a haunch or pedicel of two segments, directed forwards,
bearing two arms or rami, each composed of two segments, the outer
ramus being a little longer than the inner one. On the lower segments
in both rami of all the limbs, there is a single spine. In all the
limbs, the obliquely truncated summit of the terminal segment of the
inner ramus bears three very long, beautifully plumose spines: in the
first pair, the summit of the outer ramus bears four, and in the five
succeeding pair, six similar spines. This difference, small as it is, is
interesting, as recalling the much greater difference between the first
and succeeding pairs, in the first and second stage of development. The
terminal segments of all the rami, bearing the long plumose spines, are
directed backwards. The limbs and thorax are well furnished with
striated muscles. The animal, according to Mr. King, swims with great
rapidity, back downwards. The limbs can be withdrawn within the
carapace.

_Abdomen and Caudal Appendages._--The abdomen is small, and its
structure might easily be overlooked without careful dissection of the
different parts: it consists of three segments; the first can be seen to
be distinct from the last thoracic segment, bearing the sixth pair of
limbs, only from the fold of the epimeral element, and from its
difference in shape; the second segment is very short, but quite
distinct; the third is four or five times as long as the second, and
bears at the end two little appendages, each consisting of two segments,
the lower one with a single spine, and the upper one with three, very
long, plumose spines, like those on the rami of the thoracic limbs. The
abdomen contains only the rectum and two delicate muscles running into
the two appendages, between the bases of which the anus is seated.

_Internal Viscera._--Within the body, in front of the mouth, it was easy
to find the stomach (with two pear-shaped cæca at the upper end),
running first anteriorly, and then curving back and reaching the anus by
a long rectum, difficult to be followed: it appeared, however, to me,
that this stomach had more relation to the young Cirripede, of which
every part could now generally be traced, than to the larva, with its
closed and rudimentary mouth: the fact, however, of its being prolonged
to the anus, which is in a different position in the larva and mature
state, shows that the stomach serves, at least, as an excretory channel.
Besides the stomach, the several muscles already alluded to, and much
pulpy and oily matter, the only other internal organs consist of two
long, rather thick, gut-formed masses, into the anterior ends of which
the cement-ducts running from the prehensile antennæ could be traced.
These masses are formed of irregular orange balls, about .001 of an inch
in diameter, made up of rather large cells, so to have a grape-like
appearance, held together by a transparent pale yellowish substance, but
apparently not enclosed in a membrane: these masses lie rather
obliquely, and approach each other at their anterior ends; they extend
from above the compound eyes, to the cæca of the stomach to which they
cohere, but in young specimens, they extend some way beyond the cæca,
between the folds of the carapace. The two cement-ducts, at the points
where they enter these bodies, expand and are lost; at this point, also,
the little orange-coloured masses of cells have the appearance of being
broken down into a finer substance. Within the cement-ducts I saw a
distinct chord of rather opaque cellular matter. We shall presently see,
that these gut-formed masses are the incipient ovaria.

_The Young Cirripede within the Larva._--Several times I succeeded in
dissecting off the integuments of the lately-attached larva, and in
displaying the young _Lepas australis_ entire. The following description
applies to the Cirripede in this state; but for convenience sake, I
shall occasionally refer to its condition when a little more advanced. I
may premise, and the fact in itself is curious, that the bivalve-like
shell of the larva, together with the compound eyes, is first moulted,
and some time afterwards, the inner lining of the sack, together with
the integuments of the thorax and of the natatory legs: hence, I often
found specimens, which externally seemed to have perfected their
metamorphoses, but which, within their sacks, retained all the
characters of the natatory larva. According to Mr. King, the larva of
Lepas throws off its external shell five days after becoming attached.
Whilst the young Lepas is closely packed within the larva, the
capitulum, as known by the five valves, about equals in length the
peduncle. The peduncle occupies the anterior half of the larva; when
fully stretched, it becomes narrower and slightly longer than the
capitulum; the separation between the capitulum and peduncle is almost
arbitrary in the mature animal, and corresponds with no particular line
in the larva. Even at this early period, the muscles of the peduncle are
quite distinct. No vestige is preserved in the outer integument, of the
sternal and dorsal sutures of the larval carapace; but in the corium of
the peduncle, three coloured marks which occur near the eyes, and two
little curled marks which occur near the acoustic orifices of the larva,
are all preserved for some time after maturity. The compound eyes, as we
have seen, are attached to apodemes, springing from the sternal surface
of the larval carapace, and are consequently cast off with it: whilst
the young Cirripede is packed within the larva, the outer integument of
its peduncle necessarily forms a deep transverse fold passing over the
eyes and apodemes, and this, as we shall presently see, plays an
important part in the future position of the animal. The antennæ are not
moulted with the carapace, but left cemented to the surface of
attachment; their muscles are converted into sinewy fibres, the corium
after a short period is absorbed, and they are then preserved in a
functionless condition. No trace of the two acoustic sacks can be
perceived in the corium of the young Cirripede, excepting the coloured
marks above alluded to.

In the young capitulum, the five valves stand some way apart from each
other; they are elegant objects under the microscope; they are not
calcified, but consist exclusively of chitine; they are rather thick,
composed of an outer membrane lined by hexagonal prisms, quite unlike
any other membrane in the animal. These valves, which I have called
_primordial_ valves, resemble pretty closely in shape the valves of the
mature animal; the fork of the carina, however, is indicated only by a
slight constriction above the lower end. After the exuviation of the
larval integuments, and when calcification commences, the first layer of
shell is deposited under, and then round these primordial valves. The
latter, in well preserved old specimens, may often be detected on the
umbones of the scuta, terga, and carina, but not on the umbones of any
other valves.

The _mouth_ seems one of the earliest parts developed: in the youngest
larva dissected, I could make out at least points corresponding with
each organ; and, at the period when the young Cirripede could be
dissected out of its larval envelopes, their general details were quite
plain. The labrum, however, had not become bullate. The mouth, as we
have seen, is formed under the rudimentary mouth of the larva, and at
the same relative spot occupied by the probosciformed mouth of the larva
in the second stage. Thus far, in the young Cirripede and larva, there
has been no great change in the relative positions of the parts: the
rudimentary eyes, however, of the former are developed posteriorly to
(or above, as applied to a Cirripede,) the cast-off compound eyes of the
larva; but the position of the mouth, of the antennæ, and of the several
coloured marks in the corium, prove to demonstration, the correspondence
in both of part to part. The case is rather different with what follows.

The _Cirri_ are developed at first of considerable length, so that the
young animal may soon provide itself with food; in _Lepas australis_
they are of great length, the sixth pair consisting of seventeen or
eighteen obscure segments. The extreme tips of the twenty-four rami of
the six pair of cirri, are formed within the twenty-four, corresponding,
little, bi-segmental rami of the six pair of natatory legs; but as the
cirri are many times longer than these legs, they occupy in a bundle
the whole thorax of the larva; no part whatever of the thorax of the
Cirripede is formed within the thorax of the larva, but (together with
the pedicels of the anterior cirri) within the cephalic cavity. As a
consequence of this, the longitudinal axis of the thorax of the young
Cirripede lies almost transversely to the longitudinal axis of the
larva; and the Cirripede, from this transverse position of its thorax,
comes to be, as it were, internally, almost cut in twain, and the sack
thus produced. As soon as the young Cirripede is free and can move
itself, the cirri are curled up, and the thorax is advanced towards the
orifice of the capitulum, its longitudinal axis resuming the position of
approximate parallelism to the longitudinal axis of the whole body,
which it had in the larval condition. The reader will, perhaps,
understand what I mean, if he will look at the mature Cirripede, figured
in Pl. IX, fig. 4. In this, he will see that the body or thorax is
united to the peduncle only by a small part below the mouth; on the
other hand, if he imagines the whole bottom of the body (as high up as
the letter _h_) united and blended into the peduncle, he will see the
state in which these parts exist in the larva. Now, let him greatly
shorten the cirri, so as to resemble the natatory legs of the larva, and
then imagine a young Cirripede, with cirri _of full length_, formed
within the old one, he will see that the new thorax supporting the cirri
will have to be developed in an almost transverse position,--the animal
consequently being internally almost separated into twain.

Of the internal organs, whilst the Cirripede is still within the larva,
I have already mentioned the stomach with its pair of cæca: from the
retracted position of the thorax and rudimentary abdomen, and
consequently of the anus, compared with these parts in the larva, the
alimentary canal is not above half its former length. There is, as yet,
no trace of the filaments supposed by some to act as branchiæ, at the
base of the first pair of cirri. Nor could I perceive a trace of the
testes or vesiculæ seminales: the penis is represented by a minute,
apparently imperforate projection. I have already briefly described the
pair of large, gut-formed bodies in the larva, into the anterior ends of
which the cement-ducts ran, and evidently derived their slightly opaque,
cellular contents. At a very early age, before the young Cirripede can
be distinctly made out, the posterior ends of these gut-formed bodies
are absorbed, so as not to pass beyond the cæca of the stomach. When the
young Cirripede is plainly developed within the larva, these bodies in a
relatively reduced condition are still distinct near the cæca, and at
the opposite or anterior end (_i. e._ lower, in the position in which
Cirripedes are usually figured), they have branched out into a sheet of
delicate inosculating tubes; these could be traced by every stage,
until, in the young perfected Cirripede, they filled the peduncle as
ordinary ovarian tubes. In the larva, the two gut-formed bodies or
incipient ovaria keep of equal thickness from one to the other end, but
in the mature Cirripede, the ovarian tubes in the peduncle and the
small, glandular, grape-like masses, near the stomach-cæca, are
connected only by a delicate tube; this I failed in tracing in specimens
in the very immature condition of those now under description.

The larva fixes itself with its sternal surface parallel and close to
the surface of attachment, and the antennæ become cemented to it: if the
Cirripede, after its metamorphosis had remained in this position, the
cirri could not have been exserted, or only against the surface of
attachment; but there is a special provision, that the young Cirripede
shall immediately assume its proper position at right angles to the
position which it held whilst within the larva, namely with its
posterior end upwards. This is effected in a singular manner by the
exuviation of the great compound eyes, which we have seen are fastened
to the outer arms of the double =°UU°=-like, sternal apodemes: these
together with the eyes stretch transversely across, and internally far
up into, the body of the larva; and, as the whole has to be rejected or
moulted, the membrane of the peduncle of the young Cirripede has
necessarily to be formed with a wide and deep inward fold, extending
transversely across it; this when stretched open, after the exuviation
of the larval carapace and apodemes, necessarily causes the sternal side
of the peduncle to be longer than the dorsal, and, as a consequence,
gives to the young Cirripede its normal position, at right angles to
that of the larva when first attached.

I may here state, that I have examined the larvæ in this the final or
perfect stage in four species of Lepas, in _Conchodermavirgata_, _Ibla
quadrivalvis_, and, though rather less minutely, in _Balanus
balanoides_, and I find all essential points of organisation similar.
With the exception of diversities in the proportional sizes of the
different parts, and in the patterns on the carapace, the differences,
even in the arrangement of the spines on the limbs and antennæ, are less
than I should have anticipated.

I have in this abstract treated the metamorphoses at greater length than
I should otherwise have done, on account of the great importance of
arriving at a correct homological interpretation of the different parts
of the mature animal. In Crustacea, according to the ordinary view,
there are twenty-one segments; of these I can recognise in the
Cirripede, on evidence as good as can generally be obtained, all with
the exception of the four terminal abdominal segments; these do not
occur in any species known to me, in any stage of its development. If
that part of the larva in front of the mouth, bearing the eyes, the
prehensile antennæ, and in an earlier stage two pair of antennæ, be
formed, as is admitted in all other Crustacea, of three segments, then
beyond a doubt, from the absolute correspondence of every part, and even
every coloured mark, the peduncle of the Lepadidæ is likewise thus
formed. The peduncle being filled by the branching ovarian tubes is no
objection to this view, for I am informed on the high authority of Mr.
J. D. Dana,[12] that this is the case with the cephalo-thorax in some
true Crustaceans, for instance, in Sapphirina. To proceed, the mouth,
formed of mandibles, maxillæ, and outer maxillæ, correspond with the
fourth, fifth, and sixth segments of the archetype Crustacean.
Posteriorly to the mouth, we come, in the larva, to a rather wide
interspace without any apparent articulation or organ, and then to the
thorax, formed of six segments, bearing the six pair of limbs, of which
the first pair differs slightly from the others. The thorax is succeeded
by three small segments, differently shaped, with the posterior one
alone bearing appendages; these segments, I cannot doubt, from their
appearance alone, and from their apparent function of steering the body,
are abdominal segments. If this latter view be correct, the thoracic
segments are the six posterior ones of the normal seven segments, and
there must be two segments missing between the outer maxillæ and first
thoracic pair of legs, which latter on this view springs from the ninth
segment. Now, in a very singular Cirripede, already alluded to under the
name of Proteolepas, the two missing segments are present, the mouth
being actually succeeded by eight segments, and these by the three usual
abdominal segments,--every segment in the body being as distinct as in
an Annelid: hence in Proteolepas, adding the three segments for the
mouth and three for the carapace, we have altogether seventeen
segments, which, as I stated, is the full number ever observed in any
Cirripede, the four missing ones being abdominal, and, I presume, the
four terminal segments. That the cavity in which the thorax is lodged,
in the larva and therefore in the mature Cirripede, is simply formed by
the backward production of the carapace, does not require any
discussion. The valves have no homological signification.

   [12] This distinguished naturalist has given his opinion in the
   ‘American Journal of Science,’ March, 1846, that “the pedicel of
   Anatifa corresponds to a pair of antennæ in the young;” although
   the peduncle or pedicel is undoubtedly thus terminated, even in
   mature individuals, I think it has been shown that it is the
   whole of the anterior part of the larva in front of the mouth,
   which is directly converted into the peduncle. Professor E.
   Forbes, in his Lectures, and Professor Steenstrup, in his
   ‘Untersuchungen über das vorkommen des Hermaphroditismus in der
   Natur,’ ch. v, have considered the peduncle as a pair of fused
   legs. Lovén has taken, judging from a single sentence, the same
   view of the homologies of the external parts as I have done; in
   his description of _Alepas squalicola_, (Ofversigt of Kongl.
   Vetens., &c., Stockholm, 1844, pp. 192-4), he uses the following
   words: “Capitis reliquæ partes, ut in Lepadibus semper, in
   _pedunculum mutatæ et involucrum_,” &c.; his involucrum is the
   same as the capitulum of this work.

As we have just seen that the first pair of natatory legs is borne on
the ninth segment of the body, so it must be with the first pair of
cirri, which consequently correspond to the outer maxillipods (the two
inner pair of maxillipods or pied-machoires being here aborted) of the
higher Crustacea, and hence their difference from the five posterior
pair, which correspond with the five, ordinary pair of ambulatory legs
in these same Crustacea. The part of the body, which I have called the
prosoma, that is the protuberant, non-articulated, lower part of the
thorax (Pl. IX, fig. 4 _n_), is a special development, either of the
ninth segment, bearing the first pair of cirri, or of the segments
corresponding with the organs of the mouth. The three abdominal segments
of the larva are represented in the mature Cirripede, in the Order
containing the Lepadidæ, only by a minute, triangular gusset, let in
between the V-shaped tergal arches of the last thoracic segment: in this
gusset, small as it is, is seated the anus, and on each side the caudal
appendages, often rudimentary and sometimes absent. In another order, I
may remark, (including, probably, the Alcippe of Mr. Hancock,) the
cirri, of which there are only three pair, are abdominal.

I feel much confidence, that the homologies here given are correct. The
cause of their having been generally overlooked arises, I believe, from
the peculiar manner, already described, in which the animal, during its
last metamorphosis, is internally almost intersected: even for some
little time after discovering that the larval antennæ were always
embedded in the centre of the surface of attachment, I did not perceive,
that this was the anterior end of the whole animal. The accompanying
woodcut gives at a glance, a view of the homologies of the external
parts: the upper figure (from Milne Edwards) is a Stomapod Crustacean,
Leucifer of Vaughan Thompson, and the abdomen, which we know becomes in
Cirripedes, after the metamorphosis, rudimentary, and therefore does not
fairly enter into the comparison, is given only in faint lines: the
lower figure is a mature Lepas, with the antennæ and eyes, which are
actually present in the larva, retained and supposed to have gone on
growing. All that we externally see of a Cirripede, whether pedunculated
or sessile, is the three anterior segments of the head of a Crustacean,
with its anterior end permanently cemented to a surface of attachment,
and with its posterior end projecting vertically from it.

[Illustration: [_m._--Mouth.]]

CAPITULUM.

I will now proceed to a general description of the different parts and
organs in the Lepadidæ. The Capitulum is usually much flattened, but
sometimes broadly oval in section. It is generally formed of five or
more valves, connected together by very narrow or broad strips of
membrane; sometimes the valves are rudimental or absent, when the whole
consists of membrane. When the valves are numerous, and they
occasionally exceed a hundred in number, they are arranged in whorls,
with each valve generally so placed as to cover the interval between the
two valves above. Of all the valves, the scuta are the most persistent;
then come the terga, and then the carina; the rostrum and latera occur
only in Scalpellum and Pollicipes, and in a rudimentary condition in
Lithotrya, and, perhaps, in the fossil genus Loricula. The valves are
formed sometimes of chitine (as in Ibla and Alepas), but usually of
shell, which varies from transparency to entire opacity. The shell is
generally white, occasionally reddish or purple; exteriorly, the valves
are covered by more or less persistent, generally yellow, strong
membrane. The scuta and terga are always considerably larger than the
other valves: in the different genera the valves differ so much in shape
that little can be predicated of them in common; even the direction of
their lines of growth differs,--thus, in Lepas and some allied genera,
the chief growth of the scuta and of the carina is upwards, whereas in
Pollicipes and Lithotrya, it is entirely downwards; in Oxynaspis, and
some species of Scalpellum, it is both upwards and downwards. Even in
the same species, there is often very considerable variation in the
exact shape of the valves, more especially of the terga. The adductor
muscle is always attached to a point not far from the middle of the
scuta, and it generally has a pit for its attachment. In several genera,
namely, Pæcilasma, Dichelaspis, Conchoderma, and Alepas, the scuta show
a tendency to be bilobed or trilobed. The valves are placed either at
some distance from each other, or close together; but their growing
margins very rarely overlap each other, though this is sometimes the
case with their upper, free, tile-like apices; in a few species the
scuta and terga are articulated together, or united by a fold. The
membrane connecting the valves, where they do not touch each other, is
like that forming the peduncle, and is sometimes brilliantly coloured
crimson-red; generally, it appears blueish-gray, from the corium being
seen through. Small pointed spines, connected with the underlying
corium by tubuli, are not unfrequently articulated on this membrane: the
tubuli, however, are often present where there are no spines. To allow
of the growth of the capitulum, the membrane between the valves splits
at each period of exuviation, when a new strip of membrane is formed
beneath, connected on each side with a fresh layer of shell,--the old
and outer slips of membrane disintegrating and disappearing: when there
are many valves, the line of splitting is singularly complicated. This
membrane consists of chitine,[13] and is composed of numerous fine
laminæ. After the valves have been placed in acid, a residue, very
different in bulk in different genera, is left, also composed of
successive laminæ of chitine. It appears to me that each single lamina
of calcified chitine, composing the shell, must once have been
continuous with a non-calcified lamina in the membrane connecting the
several valves: at the line where this change in calcification
supervenes, the chitine generally assumes some colour, and becomes much
harder and more persistent; and as the whole valve is formed of
component laminæ thus edged (the once continuous laminæ of non-calcified
chitine connecting the valves, having disintegrated and disappeared) the
surfaces of the valves are generally left covered by a persistent
membrane, constituted of these edgings: this membrane has been called
the epidermis. In some genera, as in Lepas, this so-called epidermis is
seldom preserved, excepting on the last zone of growth: in Scalpellum
and Pollicipes it usually covers the whole valves. It appears to me that
the laminæ of chitine, and of calcified chitine composing the valves,
are both formed not by secretion, but by the metamorphosis of an outer
layer of corium into these substances.

   [13] Chitine is confined to the Articulata. It was Dr. C. Schmidt
   (Contributions, &c., being a Physiologico-Chemical investigation:
   in Taylor’s ‘Scientific Memoirs,’ vol. v), who discovered that
   the membrane connecting the valves and forming the peduncle, and
   the tissues of the internal animal, were composed of this
   substance. But Dr. Schmidt says that the valves in Lepas are
   composed of 3.09 of albuminates, and 96.81 of incombustible
   residue; I cannot but think that the existence of the albuminates
   is an error caused by Dr. Schmidt’s belief that the Cirripedia
   were intermediate between Crustacea and Mollusca, in the shells
   of which latter, the animal basis consists of albuminates. For
   after placing the valves of Lepas and Pollicipes in cold acid, I
   found that the membrane left could _not be dissolved_ in boiling
   caustic potash, but could, though slowly, (and without change of
   colour,) in boiling muriatic acid; and these are the main
   diagnostic characters of Chitine, compared with albuminous
   substances. I may add, that Schmidt was also induced to consider
   the shells of Cirripedia as having the same nature with those of
   Mollusca, from finding that in the above 96.81 of incombustible
   matter, 99.3 consisted of carbonate and only 0.7 of phosphate of
   lime; but Dr. Schmidt’s own analyses prove how extremely variable
   the proportions of these salts are in the Crustacea, as the
   following instance shows:--

                         _Lobster._ _Squilla._

     Phosphate of Lime     12.06      47.52
     Carbonate of Lime     87.94      52.48

   And, therefore, it is not very surprising that Cirripedia should
   have still less phosphate of lime in their shells, than has a
   lobster compared with a squilla.

Within the capitulum is the sack, which, together with the upper
internal part of the peduncle, encloses the animal’s body. The sack is
lined by a most delicate membrane of chitine, under which there is a
double layer of corium; this double layer is united together by short,
strong, transverse bundles of fibres, branched at both ends:[14] in some
genera, the ovarian tubes extend between these two layers. We have seen,
under the head of the Metamorphoses, that the delicate tunic lining the
sack is simply a duplicature of the thick membrane and valves forming
the capitulum, the whole being the posterior portion of the carapace of
the larva slightly modified.

   [14] I am much indebted to Mr. Inman of Liverpool for having
   kindly sent me excellent specimens illustrating this structure.

_Peduncle._--Its length varies greatly in different species, and even in
the same species, according to the situation occupied by the individual;
its lower end is sometimes pointed, but generally only a little narrower
than the upper end. In outline, the peduncle is usually flattened, but
sometimes quite cylindrical. It is composed of very strong, generally
thick, transparent membrane, rarely coloured reddish, and often
penetrated by numerous tubuli. The underlying corium is sometimes
coloured in longitudinal bands. At each period of growth a new and
larger integument is formed under the old one, which gradually
disintegrates and disappears; the extreme lower point is often deserted
by the corium, and ceases to grow, whilst the whole upper part still
continues increasing in diameter: in length the chief addition is made
(as is clearly seen in those genera having calcified scales), round the
upper margin, at the base of the capitulum. The surface of the membrane
is either naked or superficially clothed with minute, pointed,
articulated spines, or it is penetrated by calcified scales or styles,
(in Ibla alone formed of chitine,) which pass through it to the corium,
and are added to at their bases, like the valves, at each period of
growth. In Lithotrya alone the scales of the peduncle are moulted
together with the connecting membrane. These scales on the peduncle are
generally placed symmetrically in whorls, with each scale corresponding
with the junctions of two scales, both above and below. Except in
_Scalpellum ornatum_ and the fossil _Loricula pulchella_, they are very
small compared with the valves of the capitulum. When the scales are
symmetrical, new ones are first formed only round the summit of the
peduncle, and only those in the few uppermost whorls continue to grow or
to be added to at their bases; afterwards membrane is deposited under
them. The shelly matter of the scales resembles that of the valves, and
the manner of growth is the same; tubuli generally run to and through
them from the corium. From the continued enlargement of the membrane of
the peduncle, the scales come to stand, in the lower portion, some way
apart. In Ibla, new horny styles are formed indifferently in all parts
of the peduncle. In some species of Pollicipes, the calcareous styles
are not symmetrical or symmetrically arranged; and besides those first
formed round the top of the peduncle, there are other and larger ones
formed near its base. Lastly, in Lithotrya we have a row of calcareous
discs or an irregular, basal cup, formed in the same manner as the
valves of the capitulum: in this genus alone (as already stated,) the
calcified scales are moulted, and here alone their edges are serrated.

The peduncle is lined within by three layers of muscles, longitudinal,
transverse, and oblique, all destitute of the transverse striæ,
characteristic of voluntary muscles; they run from the bottom of the
peduncle to the base of the capitulum, as in Lepas, or half way up it,
as in Conchoderma; in Alepas alone they surround the whole capitulum up
to its summit. In Lithotrya there are two little, fan-like, transverse
muscles (involuntary), extending from the basal points of the terga to a
central line on the under side of the carina. The gentle swaying
to and fro movements, and the great power of longitudinal
contraction,--movements apparently common, as I infer from facts
communicated to me by Mr. Peach, to all the Pedunculata,--are produced
by these muscles. The interior of the peduncle is filled up with a great
mass of branching ovarian tubes; but in Ibla and Lithotrya, the upper
part of the peduncle is occupied by the animal’s body.

_Means of Attachment._--If the peduncle be very carefully removed (Tab.
IX, fig. 7 and Tab. I, fig. 6 _b_), from the surface of attachment,
quite close to the end, but not at the actual apex, the larval
prehensile antennæ can always be found: these have been sufficiently
described for our present purpose under the head of the Metamorphoses;
but I may add, that the diagnostic differences between them in the
several genera are briefly given, for a special purpose, in a discussion
on the sexes of Scalpellum at the end of that genus. We have seen in the
larva, that the cement-ducts, with their opaque cellular contents, can
be traced from within the discs of the antennæ to the anterior or lower
ends of the two gut-formed bodies, which it can be demonstrated are the
incipient ovaria.

In mature Cirripedes these ducts can be followed, in a slightly sinuous
course, along the muscles on each side within the peduncle, till they
expand into two small organs, which I have called cement-glands. These
glands are found with great difficulty, except in _Conchoderma aurita_,
where they are placed on each side under the inner layer of corium, at
the bottom of the sack, so as to be just above the top of the peduncle;
they resemble in shape a retort, (Pl. IX, fig. 3.). In _Pollicipes
mitella_ and _polymerus_ they lie half way down the peduncle, close
together, and apparently enclosed within a common membrane; in these two
species the broad end of the gland is bent towards the neck of the
retort. In Scalpellum the position is the same, but the shape is more
globular. In Ibla the structure is more simple, namely, a tube slightly
enlarged, running downwards, bent a little upwards, and then resuming
its former downward course, the lower portion forming the duct. The
gland contains a strongly coherent, pulpy, opaque, cellular mass, like
that in the cement-ducts; but in some instances, presently to be
mentioned, this cellular mass becomes converted within either the ducts
or gland, or within both, into transparent, yellow, tough cement.
Generally in Conchoderma, Pollicipes, and Scalpellum, two ovarian tubes,
but in one specimen of _Conchoderma aurita_, three tubes, and in Ibla
one tube could be seen running into or forming the gland; of the nature
of the tubes there could not be the least doubt, for at a little
distance from the glands they gave out branches (Pl. IX, fig. 3),
containing ova in every state of development. In some specimens as in
that figured of _Conchoderma aurita_, the ovarian tube on one side of
the gland is larger than on the other, and has rather the appearance of
being deeply embedded in the gland than of forming it; but, in other
specimens, the two ovarian tubes first formed a little pouch, into which
their cellular contents could be clearly seen to enter; and then this
pouch expanded into the gland; thus quite removing a doubt which I had
sometimes felt, whether the ovarian tube was not simply attached to or
embedded in the gland, without any further connection. By dissection
the multiple external coats of the gland and ovarian tubes could be seen
to be continuous. The cellular contents of the tubes passed into the
more opaque cellular contents of the gland, by a layer of transparent,
pulpy, pale, yellowish substance. There appeared in several instances to
be a relation, between the state of fulness and condition of the
contents of the gland, and of the immediately adjoining portions of the
ovarian tubes. In one specimen of _Pollicipes mitella_ it was clear that
the altered, tough, yellow, transparent, non-cellular contents of the
two glands and ducts, had actually invaded for some little distance, the
two ovarian tubes which ran into them, thus showing the continuity of
the whole. From these facts I conclude, without hesitation, that the
gland itself is a part of an ovarian tube specially modified; and
further, that the cellular matter, which in the ovarian tubes serves for
the development of the ova, is, by the special action of the walls of
the gland, changed into the opaquer cellular matter in the ducts, and
this again subsequently into that tissue or substance, which cements the
Cirripede to its surface of attachment.

As the individuals grow and increase in size, so do the glands and
cement-ducts; but it seems often to happen, that when a specimen is
immovably attached, the cementing apparatus ceases to act, and the
cellular contents of the duct become converted into a thread of
transparent tough cement; the investing membrane, also, of the ducts, in
Conchoderma sometimes becomes hard and mamillated. I have already
alluded to the case of a Pollicipes, in which both glands and ducts, and
even a small portion of the two adjoining ovarian tubes, had become thus
filled up. As in sessile Cirripedes, at every fresh period of growth a
new cement gland is formed, it has occurred to me, that possibly in
Pollicipes something similar may take place. In sessile Cirripedes, the
old cement-glands are all preserved in a functionless condition,
adhering to the membranous or calcareous basis, each new larger one
attached to that last formed, and each giving out cement-ducts, which,
bifurcating in the most complicated manner, pass outside the shell and
thus attach it to some foreign body.

The cement, removed from the outside of a Cirripede, consists of a thin
layer of very tough, bright-brown, transparent, laminated substance,
exhibiting no structure under the highest powers, or at most a very fine
dotted appearance, like a mezzotinto drawing. It is of the nature of
chitine; but boiling caustic potash has rather more effect on it than on
true chitine; and I think boiling nitric acid rather less effect. In one
single instance, namely, in Coronula, the cement comes out of the four
orifices of the two bifurcating ducts, in the shape of distinct cells,
which, between the whale’s skin and the basal membrane, arrange
themselves so as to make a circular, continuous slip of cement; then the
cells blend together, and are converted into transparent, structureless
cement. Cementing tissue or membrane would, perhaps, have been a more
correct title than cement; but, in ordinary cases, its appearance is so
little like that of an organised tissue, that I have for this reason,
and for brevity-sake, preferred the simple term of Cement.

In the larva the cement always escapes through the prehensile antennæ;
and it thus continues to do throughout life in most or all of the
species of Lepas, Conchoderma, Dichelaspis and Ibla. In the first two of
these genera, the cement escapes from the borders of the lower side of
the disc or penultimate segment of the antennæ, and can be there seen
radiating out like spokes, which at their ends divide into finer and
finer branches, till a uniform sheet of cement is formed, fastening the
antennæ and the adjoining part of the peduncle down to the surface of
attachment. In _Dichelaspis Warwickii_ and _Scalpellum Peronii_, the
cement, or part at least, comes out of the ultimate segment of the
antennæ, in the shape of one tube, within another tube of considerable
diameter and length. In _Scalpellum vulgare_, and probably in some of
the other species, which live attached to corallines, the cement soon
ceases to debouch from the antennæ, but instead, bursts through a row of
orifices on the rostral margin of the peduncle (Pl. IX, fig. 7), by
which means this margin is symmetrically fastened down to the delicate,
horny branches of the zoophyte. In Pollicipes, the two cement-ducts,
either together or separately (Pl. IX, fig. 2, 2 _a´_), wind about the
bottom of the peduncle in the most tortuous course, at each bend pouring
out cement through a hole in the membrane of the peduncle. In Ibla the
lower part of the peduncle is internally filled by cement, and thus
rendered rigid. In _Lepas fascicularis_ a vesicular ball of cement
surrounding the peduncle is thus formed (Pl. I, fig. 6), and serves as a
float! All these curious, special adaptations are described under the
respective genera. How the cement forces its way through the antennæ,
and often through apertures in the thick membrane of the peduncle, I do
not understand. I do not believe, though some appearances favoured the
notion, that the duct itself debouches and divides, at least this is not
the case in Coronula, but only that the internal chord of cellular
matter thus acts and spreads itself out; nor do I understand how, when
the antennæ and immediately adjoining parts are once cemented down, any
more cement can escape; yet this must take place, as may be inferred
from the breadth of the cemented, terminal portion of the peduncle in
Lepas and Conchoderma; and from the often active condition in old
individuals of the cementing organs.

I have entered on this subject at some length, (and I wish I had space
for more illustrations,) from its offering, perhaps, the most curious
point in the natural history of the Cirripedia. It is the one chief
character of the Sub-class. I am well aware how extremely improbable it
must appear, that part of an ovarian tube should be converted into a
gland, in which cellular matter is modified, so that instead of aiding
in the development of new beings, it forms itself into a tissue or
substance, which leaves the body[15] in order to fasten it to a foreign
support. But on no other view can the structure, clearly seen by me both
in the mature Cirripede and in the larva, be explained, and I feel no
hesitation in advancing it. I may here venture to quote the substance of
a remark made by Professor Owen, when I communicated to him the
foregoing facts, namely, that there was a new problem to solve,--new
work to perform,--to attach permanently a crustacean to a foreign body;
and that hence no one could, _a priori_, tell by what singular and novel
means this would be effected.

   [15] The protrusion of the egg-bearing pouches in Cyclops and its
   kindred genera, outside the body, offers a feeble analogy with
   what takes place in Cirripedes. Professor Allman (‘Annals of
   Natural History,’ vol. xx, p. 7,) who has attended to the
   subject, says that the external egg-bearing pouches are “a
   portion of the membrane of the true ovaries:” if the membrane of
   these pouches had been specially made adhesive, the analogy would
   have been closer.

_Filamentary Appendages._--These have generally been considered to act
as branchiæ; they occur at the bases of the first pair of cirri in
Lepas, Alepas, Conchoderma, and in three species of Pollicipes: in
Conchoderma there are similar appendages attached to the pedicels of the
cirri (Pl. IX, fig. 4, _g-k_); and in the above three species of
Pollicipes there is a double row of them on the prosoma: their numbers
differ in different species (in some there being none) of the same
genus, and even in different individuals of the same species; they are
entirely absent in the majority of the genera. These facts would
indicate that they are not of high functional importance; and they seem
so generally occupied by testes (Pl. iv, fig. 5), that I suspect their
function is quite as much to give room for the development of these
glands, as to serve for respiratory purposes. With the exception of the
four above-named genera, the mere surface of the body and of the sack
must be sufficient for respiration: in _Conchoderma aurita_ the two
great expansions of surface, afforded by the folded, tubular, ear-like
projections, aid, as I believe, towards this end.

The shape of the body varies, owing to the greater or less development
of the lower part of the prosoma, the greater or less distance of the
first from the second pair of cirri, and of the mouth from the adductor
scutorum muscle, (Pl. IX, fig. 4, and Pl. IV, 8 _a´_). In all the
genera, the body is much flattened. I may here mention a few particulars
about the muscular system. One of the largest muscular masses is formed
by the adductor scutorum, and by the muscles which surround in a double
layer (the fasciæ being oblique to each other) the whole of the upper
part of the prosoma. From under the adductor, a pair of delicate muscles
runs to the basal edge of the labrum, so as to retract the whole mouth,
and two other pair to the integument between the mouth and the adductor,
so as to fold it: again, there are other delicate muscles in some (for
instance in _Lepas Hillii_) if not in all the Lepadidæ, crossing each
other in the most singular loops, and serving apparently to fold the
membrane between the occludent edges of the scuta. Within the prosoma
there is a strong adductor muscle, running straight from side to side,
for the purpose, as it appears, of flattening the body. The thorax, on
the dorsal and ventral surfaces, is well furnished with straight and
oblique muscles (without striæ), which straighten and curl up this part
of the body. The muscles running into the pedicels of the cirri, cross
each other on the ventral surface of the thorax; the muscles within the
rami are attached to the upper segments of the pedicels. Finally, I may
remark that the whole of the body and the cirri are capable of many
diversified movements.

_Mouth._--This is prominent, and almost probosciformed (Pl. IX, fig. 4
_b_), and in the abnormal Anelasma (Pl. IV, fig. 2 _d_), quite
probosciformed,--such, also, was its character in the larval condition.
In outline, it is either sub-triangular, or oval with the longer axis
transverse; the whole is capable, as well as the separate organs, of
considerable movement, as I have seen in living sessile Cirripedes. It
is composed (Tab. V, fig. 2) of a labrum, swollen or bullate, often to
such an extent as to equal in its longitudinal axis the rest of the
mouth; of palpi soldered to the labrum; of mandibles, maxillæ, and outer
maxillæ, the latter serving as a lower lip. These organs have only their
upper segments free, but there are traces, clearly seen in the mandibles
(Pl. X, fig. 1, _a_, _b_), of their being formed of three segments. The
two lower segments are laterally united, and open into each other, the
prominence of the mouth being thus caused: this condition appears to me
curious, and is, to a certain limited extent, intermediate between those
articulated animals which have their trophi soldered into a proboscis,
and those furnished with entirely free masticatory or prehensile organs.
The palpi adhere to the corners of the labrum; and I call them palpi
only from seeing that they spring laterally from above the upper
articulation of the mandibles. The prominence of the mouth, measured
from the basal fold by which the whole is separated from the body, is
much greater on the half formed by the labrum and mandibles, than on the
other half facing the cirri. The trophi surround a cavity--the
supra-oesophageal cavity--in the middle of which, between the mandibles
is seated the orifice of the oesophagus. The oesophagus is surrounded by
long, fine, muscular fasciæ, radiating in all directions, opposing the
constrictor muscles, and is capable of violent swallowing
movements,--constriction after constriction being seen to run down its
whole course: there are also some fine muscles attached to the membrane
forming the supra-oesophageal cavity. The trophi serve merely for the
prehension of prey, and not for mastication.

The _Labrum_, as stated, is always bullate or swollen; and sometimes the
upper exterior part forms, as in Ibla (Pl. IV, fig. 8 _a_, _c_), and
Dichelaspis, an overhanging blunt point. The object, I suspect, of this
bullate form is to give, in the upper part, attachment to longer muscles
running to the lateral surfaces of the mandibles, and lower down to the
oesophagus. The crest close over the supra-oesophageal cavity, is
generally furnished with small, often bead-like teeth. The _Palpi_ are
small, their apices never actually touching each other; they are more or
less blunt, not differing much in shape in the different genera (Pl. X,
figs. 6 to 8), and clothed with spines. They are not capable of
movement; their function seems to be to prevent prey, brought by the
cirri, escaping over the labrum; I infer this from finding in Anelasma
and in the male of Ibla, which have the cirri functionless, that the
palpi are rudimentary.

The _Mandibles_ (Pl. X, figs. 1-5) have from two to ten strong teeth in
a single row; where the number exceeds five, several of the teeth are
small; the inferior angle is generally pectinated with fine spines; in
Lithotrya (fig. 2), the interspaces between the teeth are also
pectinated. In the same individual there is not unfrequently one tooth,
more or less, on opposite sides of the mouth. Internally, the mandibles
are furnished on their outer and inner sides with several ligamentous
apodemes, in Lithotrya roughened with points (Pl. X, fig. 2), for the
attachment of the muscles; of these (fig. 1), there is a chief depressor
and elevator, attached at their lower ends to near the basal fold of the
mouth, and a lateral muscle, attached to the broad basal end of the
palpi, and serving, apparently, to oppose the edge of mandible to
mandible. The _Maxillæ_ in the different genera (Pl. X, figs. 9 to 15)
differ considerably in outline; they are generally about half the size
of the mandibles; at the upper corner, there are always two or three
spines larger than the others, and often separated from them by a notch;
the rest of the spinose edge is straight, or irregular, or step-formed,
or with the lowest part projecting, or with one or two narrow
prominences bearing fine spines. All these spines, quite differently
from the teeth of the mandibles, are articulated on the edge of the
organ, and stand in a double row. At a point corresponding with the
upper articulation of the mandibles, a long, thin, narrow, rigid
apodeme, projects inwards (fig. 10), and running down nearly parallel
to the thin, outer, flexible membrane of the mouth, is attached to the
corium, and thus serves as a support to the whole organ. This apodeme is
embedded in muscles (Pl. X, fig. 10); there are other large muscles
attached to the inner side of the organ, and again others running
laterally towards the mandibles. The apodeme, of course, is moulted with
the integuments of the mouth. The _Outer Maxillæ_ (Pl. X, figs. 16, 17)
serve as a lower lip; they are thicker than the other trophi; they have
their inner surfaces clothed with spines, sometimes divided into an
upper and lower group, and occasionally separated by a deep notch: there
are often long bristles outside. They are furnished with at least two
muscles; in sessile Cirripedes I have seen that they are capable of a
rapid to and fro movement, and I have no doubt that their function is to
brush any small creature, caught by the cirri, towards the maxillæ,
which are well adapted to aid in securing the prey, and to hand it over
to the mandibles, by them to be forced down the oesophagus. On the
exterior face of the outer maxillæ, above a trace of an upper
articulation, either two small orifices or two large tubular projections
can always be discovered; and these, as will presently be mentioned, I
believe to be olfactory organs.

_Cirri._--The five posterior pair are seated close to each other and
equidistant; the first pair is generally seated at a little distance,
and sometimes at a considerable distance from the second pair. The first
pair is the shortest; the others, proceeding backwards, increase
gradually in length. The rami of each pair are either equal in length or
slightly unequal: those of the first pair are oftenest unequal. The
number of segments in the posterior cirri is sometimes very great; in
one species of Alepas, there were above sixty segments in one ramus, the
other ramus being in this unique case (Pl. X, fig. 28) small and
rudimentary. The pedicels consist of two segments, a lower, longer, and
upper short one (fig. 18, _c_, _d_.) In the usual arrangement of the
spines on the segments of the three posterior pair of cirri, there are
(figs. 26, 27) from three to six pair of long spines on the anterior
face, with generally some minute spines (occasionally forming a tuft)
intermediate between them: on the dorsal surface, in the uppermost part
of each segment, there is a tuft of short spines generally mingled with
some longer, finer ones: on the inner side of each segment, on the upper
rim, there are generally a few extremely minute and short spines. From
the increase of these latter and of the intermediate spines, the
antero-lateral faces of the segments of the first cirrus, and of the
lower segments of the anterior ramus of the second cirrus (Pl. X, fig.
25), are almost always thickly paved with brush-like masses of spines.
The lower segments of the anterior ramus of the third cirrus is
generally, though not always, thus paved: these paved segments are much
broader than the others. The posterior rami of the second and third
cirri are often in some slight degree paved, though in other cases they
resemble the three posterior pair of cirri. The two segments of the
pedicels have bristles on their anterior faces, essentially arranged on
the same plan as on the segments of the rami: the bristles are generally
not so symmetrically arranged on the pedicels of the second and third
cirri, as on the three posterior pair. There are some exceptions to the
foregoing general rules: in the posterior cirri of _Alepas cornuta_,
there is only one pair of long spines to each segment (fig. 28); in
_Dichelaspis Lowei_, there are eight pair; in _Lepas fascicularis_, in
old specimens, the segments are paved with a triangular brush of spines;
the upper segments in _Pæcilasma eburnea_ support small oblong brushes;
and, lastly, in _Pæcilasma fissa_ (fig. 29), and _crassa_, the spines
form a single circle round each segment, interrupted on the two sides.
These spines are often doubly serrated or plumose: many of them on the
protuberant segments of the first three pair of cirri, are sometimes
coarsely and doubly pectinated.

_Caudal Appendages._--These are present (Pl. X, figs. 18 to 24) seated
on each side of the anus, in all the genera, except in Conchoderma,
Anelasma, and _Scalpellum villosum_; they consist of a very small
single segment, destitute of spines in Lepas, and spinose in Pæcilasma,
Dichelaspis, Oxynaspis, Scalpellum, and some species of Pollicipes; they
consist of several segments in Alepas, Ibla, Lithotrya, and in some
species of Pollicipes. In the latter genus, some species have their
caudal appendages multiarticulate, though so obscurely articulated, that
the passage (fig. 22) from several to one segment is seen to be easily
effected. When the appendage consists of many articulations, it is
generally about as long as the pedicel of the sixth cirrus; but in _Ibla
quadrivalvis_, it is four times as long. The segments are narrow,
slightly flattened, much tapering; each (fig. 24) is surmounted by a
ring of short spines, which are generally longest on the apex of the
terminal segment. I could never trace muscles into these appendages.

_Alimentary Canal._--The oesophagus is of considerable length: it is
formed of strong, transparent, much folded membrane, continuous with the
outer integuments, and moulted with them: it is surrounded by corium,
and as already stated, by numerous muscles: at its lower end it expands
into a bell, with the edges reflexed, and sometimes sinuous: this bell
lies within the stomach, and keeps the upper broad end expanded.
According to the less or greater distance of the mouth from the adductor
muscle, the oesophagus runs in a more or less parallel course to the
abdominal surface between the first and succeeding pairs of cirri, and
enters the stomach more or less obliquely. In Ibla alone, it passes
exteriorly to, and over the adductor scutorum muscle. The stomach lies
in a much curved, almost doubled course; it is often a little
constricted where most bent; it is broadest at the upper end, and here,
in Lepas and Conchoderma, there are some deep branching cæca; in the
latter of these two genera, the whole surface is, in addition, pitted in
transverse lines. The stomach is coated by small, opaque, pulpy,
slightly arborescent glands, believed to be hepatic; these are arranged
in longitudinal lines, in all the genera, except in Alepas, in which
they are transverse and reticulated: the whole stomach is thus coated.
There is, also, a coating of excessively delicate, longitudinal and
transverse muscles without striæ. The rectum varies in length, extending
inwards from the anus to between the bases of the second and fifth pair
of cirri: it is narrow, and formed of much folded transparent membrane,
resembling the oesophagus, continuous with the outer integuments, with
which it is periodically moulted. The anus is a small longitudinal slit,
in the triangular piece of membrane representing the abdomen, let in
between the last thoracic tergal arches, as already mentioned under the
head of the Metamorphoses; it lies almost between the caudal appendages,
and opens on the dorsal surface. Within the stomach, there can generally
be plainly seen, in accordance with the period of digestion when the
specimen was taken, a thin, yet strong, perfectly transparent epithelial
membrane, not exhibiting under the highest power of the microscope any
structure: it enters the branching cæca, and extends from the edge of
the bell of the oesophagus to the commencement of the closed rectum, and
consequently terminates in a point: it consists of chitine, like the
outer integuments of the animal, and by placing the whole body in
caustic potash, I have dissolved the outer coats of the stomach, and
seen the bag open at its upper end, perfectly preserved, floating in the
middle of the body, and full of the debris of the food. In most of the
specimens which I have examined, preserved in spirits of wine, this
epithelial lining was some little way distant and separate from the
coats of the stomach; and hence was thought by M. Martin St. Ange to be
a distinct organ, like the closed tube in certain Annelids.
Occasionally, I have seen one imperfect epithelial bag or tube within
another and later-formed one. Digestion seems to go on at the same rate
throughout the whole length of the stomach; if there be any difference,
the least digested portions lie in the lower and narrower part. The
prey, consisting generally of crustacea, infusoria, minute spiral
univalves, and often of the larvæ of Cirripedes, is not triturated: when
the nutritious juices have been absorbed, the rejectamenta are cast out
through the anus, all kept together in the epithelial bag, which is
excluded like a model of the whole stomach, with the exception of that
part coated by the bell of the oesophagus. I have sometimes thought that
the bag was formed so strong, for the sake of thus carrying out the
excrement entire, so as not to befoul the sack. I believe Lepas can
throw up food by its oesophagus; at least, I found in one case, many
_half-digested_ small Crustaceans in the sack, and others of the same
kind in the stomach.

_Circulatory System._--I can add hardly anything to what little has been
given by M. Martin St. Ange: like others, I have failed, as yet, in
discovering a heart. The whole body is permeated by channels, which have
not any proper coat: there is one main channel along the ventral surface
of the thorax, dividing and surrounding the mouth, and giving out
branches which enter the inner of the two channels in each cirrus: as
Burmeister has shown, there are also two channels in the penis. There
are two dorso-lateral channels in the prosoma, which are in direct
connection with the great main channel, running down the rostral (_i.
e._, ventral) side of the peduncle. This latter main channel branches
out in the lower part, and transmits the fluid through the ovarian
tubes, whence, I believe, it flows upwards and round the sack,
re-entering the body near the sides of the adductor scutorum muscle. The
main rostral channel (or artery?) in the uppermost part of the peduncle,
has a depending curtain, which, I think, must act as a valve, so as to
prevent the circulating fluid regurgitating into the animal’s body
during the contractions of the peduncle.

_Nervous System and Organs of Sense._--In most of the genera, there are
six _main_ ganglia, namely, the supra-oesophageal, and five thoracic
ganglia; but in _Pollicipes mitella_ there are only four thoracic
ganglia. Of these, the first thoracic or infra-oesophageal ganglion is
considerably the largest and most massive; it is squarish, or oval, or
heart-shaped; it presents no trace of being formed by the union of two
lateral ganglia. Two great nerves spring from its under side (A),
represented in the woodcut on page 49, by dotted lines, and run straight
down amongst the viscera in the prosoma: these nerves are about as large
as those forming the collar and those running to the second ganglion;
hence, six great nerves meet here, two in front, two behind, and two on
the under side. At the anterior end, over the junction with the collar
chord, three equal-sized nerves rise on each side, with a fourth,
smaller one, outside; these go to the trophi and to the two olfactory
sacks. At the posterior end, on each side, a pair of nerves branch out
rectangularly, one of which (_a_,) goes to the first cirrus, and there
divides into two branches; of these, the upper runs up the cirrus, and
the lower one downwards. The other nerve (_b_), proceeding on each side
from this first thoracic ganglion, runs to the muscles beneath the basal
articulation of the first cirrus. The collar surrounding the oesophagus
is generally very long, sometimes equalling the whole thoracic chord; at
a middle point, a small branch is sent off, and at the anterior end
(_e_, _e_), close to the supra-oesophageal ganglia, double or treble
fine branches run to the true ovaria, lying close to the upper end of
the stomach. The four (or only three) other thoracic ganglia, when
viewed as transparent bodies, are seen to be solid; but in some of the
genera, as in Conchoderma, the outline plainly shows, that each consists
of a lateral pair fused together. The second thoracic ganglion (B) is
rather small; it is either close to the first, as in _Pollicipes
mitella_ and _Lepas fascicularis_, or far distant, as in Ibla. The third
(C) and fourth are of about the same size with the second: these three
ganglia send large branches to the second, third, and fourth pair of
cirri: other minute branches spring from their under sides, and from the
intermediate double chords. The fifth ganglion is larger and longer than
the three preceding ones, and gives off nerves to the fifth and sixth
pair of cirri; it is clearly formed by the union of the fifth, with what
ought to have formed a sixth ganglion. The two nerves going to the sixth
cirrus give off on their inner sides, each a great branch to the penis.
In _Pollicipes mitella_, in which there are only four instead of five
thoracic ganglia, it is evident from the outline and position of the
nerves going to the fourth pair of cirri, that the fourth ganglion is
fused into the fifth, itself, as we have just seen, normally composed of
two consecutive ganglia. In this Pollicipes there is other evidence of
concentration in the nervous system, for none of the ganglia show signs
of being formed of lateral pairs; the second is close to the first; and
the abdominal double chord is in part separated by a mere cleft; lastly,
as we shall immediately see, the same remark is applicable to the
supra-oesophageal ganglia.

The latter (D) alone remain to be described; they present far more
diversity in shape than do the thoracic ganglia; they are almost always
seen in outline to be laterally distinct, and usually resemble two pears
with their tapering ends cut off and united; in a transverse line they
are as long as the infra-oesophageal ganglion, but are much less
massive. In _Lepas fascicularis_ (D), they are pear-shaped; in
_Pollicipes mitella_ they are globular, and separated by a third
globular ganglion, which I believe is the ophthalmic ganglion, presently
to be described; in _Pollicipes spinosus_, however, the ophthalmic
ganglion is, as usual, placed in advance of the supra-oesophageal
ganglion, which latter, in this one species, shows no sign of being
formed of a lateral pair fused together. In _Alepas cornuta_ the
supra-oesophageal ganglion consists of two quite distinct ganglia,
elongated in the longitudinal axis of the body, and separated from each
other by the whole width of the mouth; the chord which unites them is of
the same thickness as the rest of the collar. In all the genera, from
the front of each of the two supra-oesophageal ganglia, a pair of
nerves, (_f_, _f_,) united and together as large as the collar nerve,
rises, and can be traced running unbranched, in a nearly straight line,
for a length equalling the whole rest of the nervous chord, so as to
supply the peduncle and the inside of the capitulum or sack. At the
inner ends of these two same ganglia, from a central point where they
are united, a little central branch runs in front to the adductor
scutorum and other adjoining muscles; and still smaller fibrils run
behind to the oesophageal muscles.

[Illustration: Diagram of the anterior portion of the nervous system in
_Lepas fascicularis_. A. First thoracic or infra-oesophageal ganglion.
B. Second thoracic. C. Third thoracic ganglion. D. Supra-oesophageal
ganglion. E. The two ophthalmic ganglia. F. Double eye. _a_. Nerve going
to first cirrus; _b_, to the muscles below the first cirrus; _c_, to the
second cirrus; _d_, to the third; _e_, nerves running to the ovaria;
_f_, double nerves supplying the sack and peduncle.]

_Ophthalmic Ganglia and Eyes._--Owing to Professor Leidy’s[16] discovery
of eyes in a Balanus, I was led to look for them in the Lepadidæ.
Extending from the front of the two supra-oesophageal ganglia, two
chords may be seen in _Lepas fascicularis_ (of which a rude diagram is
here given), to run into two small, perfectly distinct oval ganglia
(E), which are not united by any transverse commissure. From the
opposite ends of these two ganglia smaller nerves run, and, bending
inwards at right angles, enter, beyond the middle, an elongated (F),
almost black, eye, composed of two eyes united together. Although in
outline the eye appears single, two lenses can be distinctly seen at the
end, directed upwards and towards the ganglia; two pigment-capsules can
also be distinguished; these are deep and cup-formed, and of a dark
reddish-purple. The following measurements will show the proportions of
the parts in a specimen of the _Lepas fascicularis_ having a capitulum
4/10ths of an inch in length.

  Double eye { length                         26/6000
             { width                          13/6000

  Diameter of single lens                      6/6000

  Ophthalmic ganglion { length                16/6000
                      { breadth               11/6000

  Supra-oesophageal ganglion, }
  transverse or longest axis    }            126/6000
  of both together              }

  Supra-oesophageal ganglion, }
  longitudinal axis of          }             45/6000

  Infra-oesophageal ganglion, }
  transverse axis of            }            120/6000

  Infra-oesophageal ganglion, }
  longitudinal axis of          }            114/6000

   [16] Proceedings of the Academy of Natural Sciences,
   Philadelphia. No. i, vol. iv, Jan. 1848.

In _Conchoderma aurita_ the ophthalmic ganglia are much smaller, and
nearer to the supra-oesophageal ganglion, than in _L. fascicularis_. In
_Alepas cornuta_ the ophthalmic chords run towards each other from the
two distant and separate supra-oesophageal ganglia; and the ophthalmic
ganglia, (instead of being quite separate, as in _L. fascicularis_,) are
united by their front ends, and the two eyes instead of standing some
way in front, with nerves running to them, are embedded on the double
ophthalmic ganglion; the pigment-capsules here, also, have the shape of
mere saucers, and are joined back to back, with the two lenses
projecting far out of them. In neither sex of Ibla could I perceive that
the eye was double. In _Pollicipes spinosus_ the ophthalmic ganglion
stands in front of the single supra-oesophageal ganglion, and shows no
signs of being formed of a lateral pair; the eyes themselves, however,
differently from, in all the foregoing cases, are, though approximate,
quite distinct. In _Pollicipes mitella_ I did not see the eyes; but the
ophthalmic ganglion consists, as I believe, of a single globular one,
placed exactly between the two globular, supra-oesophageal ganglia, all
three being of nearly equal size. Professor Leidy does not mention the
ophthalmic ganglia; hence I infer that in Balanus, which is a more
highly organised Cirripede, they are fused into the supra-oesophageal
ganglion.

In all the genera, the double eye is seated deep within the body; it is
attached by fibrous tissue to the radiating muscles of the lowest part
of the oesophagus, and lies actually on the upper part of the stomach;
consequently, a ray of light, to reach the eye, has to pass through the
exterior membrane and underlying corium connecting the two scuta, and to
penetrate deeply into the body. In living sessile Cirripedes, vision
seems confined to the perception of the shadow of an object passing
between them and the light; they instantly perceived a hand passed
quickly at the distance of several feet between a candle and the basin
in which they were placed.

As the infra-oesophageal ganglion sends nerves to the trophi and to the
first pair of cirri, it must correspond to the segments, from the fourth
to the ninth inclusive, of the archetype crustacean. The state of the
supra-oesophageal and ophthalmic ganglia appears to me very interesting:
I do not believe that in any _mature_ ordinary crustacean, the first or
ophthalmic ganglion can be shown to be distinct from the two succeeding
ganglia, or to be itself composed of a pair laterally distinct. The
ganglia, corresponding with the second and third segments of the body,
which should normally support two pair of antennæ, are in the Lepadidæ
united together; but laterally they are generally distinct in outline,
and are actually separate in Alepas: the supra-oesophageal ganglion
shows also its double nature, by giving rise to a pair of large double
nerves, evidently corresponding with the two pair of antennular nerves
in ordinary crustaceans. The embryonic condition of the whole
supra-oesophageal portion of the nervous system in the Lepadidæ,
corresponds with the rudimentary state of the only organ of sense
supplied by it, namely, the eye, which in size and general appearance
has retrograded to the state in which it was in, during the first stage
of development of the larva;--I have used the term embryonic, because,
in the embryos of ordinary crustacea, all the ganglia are at first
longitudinally distinct, and laterally quite separate. The conclusion at
which we before arrived from studying the metamorphoses, namely, that
the whole peduncle and capitulum consisted of the first three segments
of the head, is beautifully supported by the structure of the nervous
system, in which these parts are seen to be supplied with nerves
exclusively from the supra-oesophageal ganglion: now in ordinary
crustacea the supra-oesophageal ganglion sends nerves to the eyes and
the two pair of antennæ corresponding, as is known by embryological
dissections, to the first three segments of the body. Moreover, it is
asserted that the carapace which covers the thorax in crustacea, is not
formed by the development of the first segment; and this, likewise, may
be inferred to be the case with the peduncle and capitulum in the
Lepadidæ, as the nerves of the ophthalmic ganglia go exclusively to the
eyes. Finally, I may remark that in Pollicipes, looking to the whole
nervous system, the state of concentration nearly equals that in certain
macrourous decapod crustaceans, for instance the _Astacus marinus_, of
which a figure is given by Milne Edwards.

_Olfactory Organs._--In the outer maxillæ, at their bases where united
together, but above the basal fold separating the mouth from the body,
there are, in all the genera, a pair of orifices (Pl. X, fig. 16); these
are sometimes seated on a slight prominence, as in Lithotrya, or on the
summit of flattened tubes (Pl. X, fig. 17), projecting upwards and
towards each other, as in Ibla, Scalpellum, and Pollicipes. In Ibla
these tubular projections rise from almost between the outer and inner
maxillæ. It is impossible to behold these organs, and doubt that they
are of high functional importance to the animal. The orifice leads into
a deep sack lined by pulpy corium, and closed at the bottom. The outer
integument is inflected inwards, (hence periodically moulted,) and
becoming of excessive tenuity, runs to near the bottom of the sack,
where it ends in an open tube: so excessively thin is this inflected
membrane, that, until examining Anelasma, I was not quite certain that I
was right in believing that the outer integument did not extend over the
whole bottom. I several times saw a nerve of considerable size entering
and blending into a pulpy layer at the bottom of the sack of corium; but
I failed in tracing to which of the three pair of nerves, springing from
the front end of the infra-oesophageal ganglion, it joined. I can hardly
avoid concluding, that this _closed_ sack, with its naked bottom, is an
organ of sense; and, considering that the outer maxillæ serve to carry
the prey entangled by the cirri towards the maxillæ and mandibles, the
position seems so admirably adapted for an olfactory organ, whereby the
animal could at once perceive the nature of any floating object thus
caught, that I have ventured provisionally to designate the two orifices
and sacks as olfactory.

_Acoustic_ (?) _Organs._--A little way beneath the basal articulation of
the first cirrus (Pl. IX, fig. 4 _d_, and Pl. IV, fig. 2 _e_), on each
side, there may be seen a slight swelling, and on the under side of
this, a transverse slit-like orifice, 1/20th of an inch in length in
Conchoderma, but often only half that size. In Ibla this orifice is
seated lower down (Pl. IV, fig. 8 _a´_, _e_), between the bases of the
first and second cirri, which are here far apart: in _Alepas cornuta_ it
is placed rather nearer to the adductor scutorum muscle, namely, beneath
the mandibles. The orifice leads into a rather deep and wide meatus; the
external integument is turned in for a short distance, widening a
little, and then ends abruptly. The meatus, enlarging upwards, is lined
by thick pulpy corium, and is closed at the upper end; from its summit
is suspended a flattened sack of singular and different shapes in the
different genera. This, the so-called acoustic sack of _Conchoderma
virgata_, is figured Pl. IX, fig. 6. The deep and wide notch faces
towards the posterior end of the animal; the inferior lobe, thus almost
cut off, is flattened in a different plane from the upper part; the lobe
is lodged in a little pouch of corresponding form, leading from the open
meatus in which the upper part is included. In _Conchoderma aurita_, the
top of the acoustic sack is narrower and more constricted, the whole
more rounded, and the lobe more turned down. In _Lepas fascicularis_ the
notch is not so deep or wide, and the lobe larger. In _Ibla Cumingii_
the sack is of the shape of a vase, with one corner folded over. In
_Scalpellum vulgare_ it is small, oval, with the lower end much pushed
in, and furnished with a little crest. Lastly, in _Pollicipes mitella_
it is simply oval. In all cases the sack is empty, or contains only a
little pulpy matter: it consists of brownish, thick, and remarkably
elastic tissue, formed, apparently, of transverse little pillars,
becoming fibrous on the outside, and with their inner ends appearing
like hyaline points. The mouth of the acoustic sack (removed in the
drawing) is closed by a tender diaphragm, through which I saw what I
believe was a moderately-sized nerve enter; I have not yet succeeded in
tracing this nerve. The first pair of cirri seem, to a certain extent,
to serve as antennæ, and therefore the position of an acoustic organ at
their bases, is analogous to what takes place in crustacea; but there
are not here any otolites, or the siliceous particles and hairs, as
described by Dr. Farre, in that class. Nevertheless, the sack is so
highly elastic, and its suspension in a meatus freely open to the water,
seems so well adapted for an acoustic organ, that I have provisionally
thus called it. In the larva, as I have shown, a pouch, certainly
serving for some sense, I believe for hearing, is seated in quite a
different position at the anterior end of the carapace. I may mention
that I found sessile Cirripedes very sensitive of vibrations in objects
adjoining them, though not, apparently, of noises in the air or water.
In a group of specimens, I could not touch one even most delicately
with a needle, without all the adjoining ones instantly withdrawing
their cirri; it made no difference if the one touched had its operculum
already closed and motionless.

_Reproductive System_,--_Male Organs._--All the Cirripedia which I have
hitherto examined, with the exception of certain species of Ibla and
Scalpellum, are hermaphrodite or bisexual.[17] I shall so fully describe
the sexual relations of the several species of these two genera, under
their respective headings, and at the end of the genus of Scalpellum,
that I will not here give even an abstract of the grounds on which my
firm belief is based, that the masculine power of certain hermaphrodite
species of Ibla and Scalpellum, is rendered more efficient by certain
parasitic males, which, from their not pairing, as in all hitherto known
cases, with females, but with hermaphrodites, I have designated
_Complemental Males_.

   [17] I am compelled to differ greatly from the account given by
   Prof. Steenstrup of the reproductive system in the Cirripedia, in
   his ‘Untersuchungen über das Vorkommen des Hermaphroditismus, ch.
   v, 1846;--a translation of which I have seen, owing to the great
   kindness of Mr. Busk. Mr. Goodsir has described (‘Edin. New Phil.
   Journal,’ July 1843,) what he considers the male of Balanus; but
   I have seen this same parasitic creature charged with ova,
   including larvæ! From the resemblance of the larvæ to the little
   crustacean described by Mr. Goodsir, in the same paper, as a
   distinct parasite, I believe the latter to be the male of his
   so-called male Balanus, and that all belong to the same species,
   allied to Bopyrus. This genus, as is well known, is parasitic on
   other crustacea; and it is a rather interesting fact thus to
   find, that this new parasite which is allied to Bopyrus, in
   structure, is likewise allied to it in habits, living attached to
   Cirripedia, a sub-class of the crustacea.

The male organs have been well described by M. Martin St. Ange, whose
observations have since been confirmed by R. Wagner.[18] The testes are
small, often leaden-coloured, either pear or finger-shaped, or branched
like club-moss,--these several forms sometimes occurring in the same
individual; they coat the stomach, enter the pedicels, and even the
basal segments of the rami of the cirri, and in some genera occupy
certain swellings on the thorax and prosoma, and in others the
filamentary appendages: the testes seen in the apex in one of these
appendages in Conchoderma, is represented in Pl. IX, fig. 5. The two
vesiculæ seminales are very large; they lie along the abdominal surface
of the thorax, and generally (but not in some species of Scalpellum)
enter the prosoma, where their broad ends are often reflexed; here the
branched vessels leading from the testes enter. The membrane of the
vesiculæ seminales is formed of circular fibres; and is, I presume,
contractile, for I have seen the spermatozoa expelled with force from
the cut end of a living specimen. The two canals leading from the
vesiculæ generally unite in a single duct at the base of the penis; but
in _Conchoderma aurita_, half-way up it. The probosciformed penis,
except in certain species of Scalpellum, is very long; it is capable of
the most varied movements; it is generally hairy, especially at the end;