(Ovularia); we include among them all _single-celled animals_, all
animals whose body, in the fully developed state, possesses the
form-value of a _simple plastid_ (of a cytod or a cell), also those
simple animal forms whose body consists of an aggregation of several
cells perfectly similar one to another.

The _Archaic animals_ (Archezoa) form the first class in the series of
Egg animals. It contains only the most simple and most ancient primary
forms of the animal kingdom, whose former existence we have proved by
means of the fundamental law of biogenesis; they are, (1) Animal Monera;
(2) Animal Amœbæ; (3) Animal Synamœbæ. We may, if we choose, include
among them a portion of the still living Monera and Amœbæ, but another
portion (according to the discussion in Chapter XVI.) must on account of
their neutral nature be considered as Protista, and a third portion, on
account of their vegetable nature, must be considered as plants.

A second class of the egg animals consists of the _Gregarines_
(Gregarinæ), which live as parasites in the intestines and body-cavities
of many animals. Some of these Gregarines are perfectly simple cells
like the Amœbæ; some form chains of two or three identical cells, one
lying behind the other. They differ from the naked Amœbæ by possessing
a thick, simple membrane, which surrounds their cell-body; they can be
considered as animal Amœbæ which have adopted a parasitical mode of
life, and in consequence have surrounded themselves with a secreted
covering.

As a third class of egg animals, we adopt the real _Infusoria_
(Infusoria), embracing those forms to which modern zoology almost
universally limits this class of animals. The principal portion of them
consists of the small _ciliated Infusoria_ (Ciliata), which inhabit all
the fresh and salt waters of the earth in great numbers, and which swim
about by means of a delicate garb of vibratile fringes. A second and
smaller division consists of the adherent _sucking Infusoria_ (Acinetæ),
which take their food by means of fine sucking-tubes. Although during
the last thirty years numerous and very careful investigations have been
made on these small animalcules,—which are mostly invisible to the naked
eye,—still we are even now not very sure about their development and
form-value. We do not even yet know whether the Infusoria are single or
many-celled; but as no investigator has as yet proved their body to be a
combination of cells, we are, in the mean time, justified in considering
them as single-celled, like the Gregarines and the Amœbæ.

The second main class of primæval animals consists of the _Germ animals_
(Blastularia). This name we give to those extinct Protozoa which
correspond to the two ontogenetic embryonic forms of the six higher
animal tribes, namely, the Planula and the Gastrula. The body of these
Blastularia, in a perfectly developed state, was composed of many cells,
and these cells moreover differentiated—in two ways at least—into an
external (animal or dermal) and an internal (vegetative or gastral)
mass. Whether there still exist representatives of this group is
uncertain. Their former existence is undoubtedly proved by the two
exceedingly important ontogenetic animal forms which we have already
described as Planula and Gastrula, and which still occur as a transient
stage of development in the ontogeny of the most different tribes of
animals. Corresponding to these, we may, according to the biogenetic
principle, assume the former existence of two distinct classes of
Blastularia, namely, the _Planæada_ and _Gastræada_. The type of the
_Planæada_ is the _Planæa_—long since extinct—but whose historical
portrait is still presented to us at the present day in the widely
distributed _ciliated larva_ (Planula). (Frontispiece, Fig. 4.) The type
of the _Gastræada_ is the _Gastræa_, of whose original nature the
mouth-and-stomach larva (Gastrula), which recurs in the most different
animal tribes, still gives a faithful representation. (Frontispiece Fig.
5, 6.) Out of the Gastræa, as we have previously mentioned, there were
at one time developed two different primary forms, the Protascus and
Prothelmis; the former must be looked upon as the primary form of the
Zoophytes, the latter as the primary form of Worms. (Compare the
enunciation of this hypothesis in my Monograph of the Calcareous
Sponges, vol i. p. 464.)

The _Animal-plants_ (Zoophyta, or Cœlenterata) which constitute the
second tribe of the animal kingdom, rise considerably above the
primitive animals in the characters of their whole organisation, while
they remain far below most of the higher animals. For in the latter
(with the exception only of the lowest forms) the four distinct
functions of nutrition—namely, digestion, circulation of the blood,
respiration, and excretion—are universally accomplished by four
perfectly different systems of organs: by the intestines, the vascular
system, the organs of respiration, and the urinary apparatus. In
Zoophytes, however, these functions and their organs are not yet
separate, and are all performed by a single system of alimentary canals,
by the so-called gastro-vascular system, or the cœlenteric apparatus of
the intestinal cavity. The mouth, which is also the anus, leads into a
stomach, into which the other cavities of the body also open. In
Zoophytes the body-cavity, or “cœloma,” possessed by the four higher
tribes of animals is still completely wanting, likewise the vascular
system and blood, as also the organs of respiration, etc.

All Zoophytes live in water; most of them in the sea, only a very few in
fresh water, such as fresh-water sponges (Spongilla) and some primæval
polyps (Hydra, Cordylophora). A specimen of the pretty flower-like forms
which are met with in great variety among Zoophytes is given on Plate
VII. (Compare its explanation in the Appendix.)

The tribe of animal-plants, or Zoophytes, is divided into two distinct
provinces, the _Sponges_, or _Spongiæ_, and the _Sea-nettles_, or
_Acalephæ_ (p. 144). The latter are much richer in forms and more highly
organized than the former. In all Sponges the entire body, as well as
the individual organs, are differentiated and perfected to a much less
extent than in Sea-nettles. All Sponges lack the characteristic
_nettle-organs_ which all Sea-nettles possess.

The common primary form of all Zoophytes must be looked for in the
_Protascus_, an animal form long since extinct, but whose existence is
proved according to the biogenetic principle by the Ascula. This Ascula
is an ontogenetical development form which, in Sponges as well as in
Sea-nettles, proceeds from the Gastrula. (Compare the Ascula of the
calcareous sponge on the Frontispiece, Fig. 7, 8.) For after the
Gastrula of zoophytes has for a time swum about in the water it sinks to
the bottom, and there adheres by that pole of its axis which is opposite
to the opening of the mouth. The external cells of the ectoderm draw in
their vibrating, ciliary hairs, whereas, on the contrary, the inner
cells of the entoderm begin to form them. Thus the Ascula, as we call
this changed form of larva, is a simple sack, its cavity (the cavity of
the stomach or intestine) opening by a mouth externally, at the upper
pole of the longitudinal axis (opposite the basal point of fixture). The
entire body is here in a certain sense a mere stomach or intestinal
canal, as in the case of the Gastrula. The wall of the sack, which is
both body wall and intestinal wall, consists of two layers or coats of
cells, a fringed _entoderm_, or gastral layer (corresponding with the
inner or vegetative germ-layer of the higher animals), and an unfringed
exoderm or dermal layer (corresponding with the external or animal
germ-layer of the higher animals). The original _Protascus_, a true
likeness of which is still furnished by the Ascula, probably formed
egg-cells and sperm-cells out of its gastral layer.

The Protascads—as we will call the most ancient group of vegetable
animals, represented by the Protascus-type—divided into two lines or
branches, the Spongiæ and the Sea-nettles, or Acalephæ. I have shown in
my Monograph of the Calcareous Sponges (vol. i. p. 485) how closely
these two main classes of Zoophytes are related, and how they must both
be derived, as two diverging forms, from the Protascus-form. The primary
form of Spongiæ, which I have there called Archispongia, arose out of
the Protascus by the formation of pores through its body-wall; the
primary form of Sea-nettles, which I there called Archydra, developed
out of the Protascus by the formation of nettle-organs, as also by the
formation of feelers or tentacles.

The main-class or branch of the _Sponges_, _Spongiæ_, or _Porifera_,
lives in the sea, with the single exception of the green fresh-water
Sponge (Spongilla). These animals were long considered as plants, later
as Protista; in most Manuals they are still classed among the primæval
animals, or Protozoa. But since I have demonstrated their development
out of the Gastrula, and the construction of their bodies of two
cellular germ-layers (as in all higher animals), their close
relationship to Sea-nettles, and especially to the Hydrapolyps, seems
finally to be established. The _Olynthus_ especially, which I consider
as the common primary form of calcareous sponges, has thrown a complete
and unmistakable light upon this point.

The numerous forms comprised in the class of Spongiæ have as yet been
but little examined; they may be divided into three legions and eight
orders. The first legion consists of the soft, gelatinous _Mucous
Sponges_ (Myxospongiæ), which are characterized by the absence of any
hard skeleton. Among them are, on the one hand, the long-since-extinct
primary forms of the whole class, the type of which I consider to be the
Archispongia; on the other hand there are the still living, gelatinous
sponges, of which the _Halisarca_ is best known. We can obtain a notion
of the Archispongia, the most ancient primæval sponge, if we imagine the
Olynthus (see Frontispiece), to be deprived of its radiating calcareous
spiculæ.

The second legion of Spongiæ contains the _Fibrous Sponges_
(Fibrospongiæ), the soft body of which is supported by a firm, fibrous
skeleton. This fibrous skeleton often consists merely of so-called
“horny fibres,” formed of a very elastic, not readily destructible,
organic substance. This is the case for instance in our common bathing
Sponge (Euspongia officinalis), the purified skeleton of which we use
every morning when washing. Blended with the horny, fibrous skeleton of
many of these Sponges, there are numerous flinty spicula; this is the
case for example with the fresh-water Sponge (Spongilla). In others the
whole skeleton consists of only calcareous or silicious spicula which
are frequently interwoven into an extremely beautiful lattice-work, as
in the celebrated Venus’ Flower Basket (Euplectella). Three orders of
fibrous sponges may be distinguished according to the different
formation of the spicula, namely, Chalynthina, Geodina, and
Hexactinella. The natural history of the fibrous sponges is of especial
interest to the Theory of Descent, as was first shown by Oscar Schmidt,
the greatest authority on this group of animals. In no other group,
perhaps, can the unlimited pliability of the specific form, and its
relation to Adaptation and Inheritance, be so clearly followed step by
step; perhaps in no other group is the species so difficult to limit and
define.

[Illustration: Pl. VII.

E. Haeckel del. Lagesse sc.]

This proposition, which applies to the great legion of the Fibrous
Sponges, applies in a still higher degree to the smaller but exceedingly
interesting legion of the calcareous sponges (Calcispongiæ), on which in
1872, after five years’ careful examination, I published a comprehensive
Monograph. The sixty plates of figures accompanying this Monograph
explain the extreme pliability of these small sponges “good species” of
which, in fact, cannot be spoken of in the usual systematic sense. We
find among them only varying series of forms, which do not even
completely transmit their specific form to their nearest descendants,
but by adaptation to subordinate, external conditions of existence,
perpetually change. It frequently occurs here, that there arise out of
one and the same stock different form-species, which according to the
usual system would belong to several quite distinct genera; this is the
case, for instance, with the remarkable Ascometra (Frontispiece, Fig.
10.) The entire external bodily form is much more pliable and protean in
Calcareous Sponges than in the silicious sponges, which are
characterized by possessing silicious spicula, forming a beautiful
skeleton. Through the study of the comparative anatomy and ontogeny of
calcareous sponges, we can recognise, with the greatest certainty, the
common primary form of the whole group, namely, the sack-shaped
_Olynthus_, whose development is represented in the Frontispiece
(compare its explanation in the Appendix). Out of the Olynthus (Fig. 9
on the Frontispiece), the order of the Ascones was the first to develop,
out of which, at a later period, the two other orders of Calcareous
Sponges, the _Leucones_ and _Sycones_, arose as diverging branches.
Within these orders, the descent of the individual forms can again be
followed step by step. Thus the Calcareous Sponges in every respect
confirm the proposition which I have elsewhere maintained: that “the
natural history of sponges forms a connected and striking argument in
favour of Darwin.”

The second main class or branch in the tribe of Zoophytes is formed by
the Sea-nettles (Acalephæ, or Cnidæ). This interesting group of animals,
so rich in forms, is composed of three different classes, namely, the
Hood-jellies (Hydromedusæ), the Comb-jellies (Ctenophora), and the
Corals (Coralla). The hypothetical, extinct Archydra must be looked upon
as the common primary form of the whole group; it has left two near
relations in the still living fresh-water polyps (Hydra and
Cordylophora). The Archydra was very closely related to the simplest
forms of Spongiæ (Archispongia and Olynthus), and probably differed from
them only by possessing nettle organs, and by the absence of cutaneous
pores. Out of the Archydra there first developed the different Hydroid
polyps, some of which became the primary forms of Corals, others the
primary forms of Hydromedusæ. The Ctenophora developed later out of a
branch of the latter.

The Sea-nettles differ from the Spongiæ (with which they agree in the
characteristic formation of the system of the alimentary canal)
principally by the constant possession of nettle organs. These are small
bladders filled with poison, large numbers—generally millions—of which
are dispersed over the skin of the sea nettles, and which burst and
empty their contents when touched. Small animals are killed by this; in
larger animals this nettle poison causes a slight inflammation of the
skin, just as does the poison of our common nettles. Any one who has
often bathed in the sea, will probably have at times come in contact
with large Hood-jellies (Jelly-fish), and become acquainted with the
unpleasant burning feeling which their nettle organs can produce. The
poison in the splendid blue Jelly-fish, Physalia, or Portuguese
Man-of-war, acts so powerfully that it may lead to the death of a human
being.

The class of Corals (Coralla) lives exclusively in the sea, and is more
especially represented in the warm seas by an abundance of beautiful and
highly-coloured forms like flowers. Hence they are also called
_Flower-animals_ (Anthozoa). Most of them are attached to the bottom of
the sea, and contain an internal calcareous skeleton. Many of them by
continued growth produce such immense stocks that their calcareous
skeletons have formed the foundation of whole islands, as is the case
with the celebrated coral reefs and atolls of the South Seas, the
remarkable forms of which were first explained by Darwin.(13) In corals
the counterparts, or antimera—that is, the corresponding divisions of
the body which radiate from and surround the central main axis of the
body—exist sometimes to the number of four, sometimes to the number of
six or eight. According to this we distinguish three legions, the
Fourfold (Tetracoralla), Sixfold (Hexacoralla), and Eightfold corals
(Octocoralla). The fourfold corals form the common primary group of the
class, out of which the sixfold and eightfold have developed as two
diverging branches.

SYSTEMATIC SURVEY

_Of the 4 Classes and 30 Orders of the Animal Plants, or Zoophytes._

 ----------------+--------------------+--------------------+---------------
  _Class of the_ | _Legions of the_   | _Orders of the_    | _A Genus Name_
  _Zoophytes._   |  _Zoophytes._      | _Zoophytes._       |  _as example._
 ----------------+--------------------+--------------------+---------------
        I.        { I. Myxospongiæ      { 1. Archispongina | Archispongia
                  { _Mucous Sponges_    { 2. Halisarcina   | Halisarca
    =Sponges=     {                                        |
                  { II. Fibrospongiæ    { 3. Chalynthina   | Spongilla
    +Spongiæ+     {_Fibrous Sponges_    { 4. Geodina       | Ancorina
        or        {                     { 5. Hexactinella  | Euplectella
    +Porifera+    {                                        |
                  { III. Calcispongiæ   { 6. Ascones       | Olynthus
                  {_Calcareous Sponges_ { 7. Leucones      | Dyssycus
                  {                     { 8. Sycones       | Sycurus
       II.        { IV. Tetracoralla    { 9. Rugosa        | Cyathophyllum
                  {_Fourfold Corals_    { 10. Paranemeta   | Cereanthus
    =Corals=      {                                        |
                  { V. Hexacoralla      { 11. Cauliculata  | Antipathes
    +Coralla+     {_Sixfold Corals_     { 12. Madreporaria | Astræa
        or        {                     { 13. Halirhoda    | Actinia
    +Anthozoa+    {                                        |
                  { VI. Octocoralla     { 14. Alcyonida    | Lobularia
                  {_Eightfold Corals_   { 15. Gorgonida    | Isis
                  {                     { 16. Pennatulida  | Veretillum
      III.        { VII. Archydræ       } 17. Hydraria     | Hydra
                  {_Primæval Polyps_    }                  |
  =Jelly-polyps=  {                                        |
                  { VIII. Leptomedusæ   { 18. Vesiculata   | Sertularia
   +Hydromedusæ+  {_Soft Jelly-fish_    { 19. Ocellata     | Tubularia
                  {                     { 20. Siphonophora | Physophora
        or        {                                        |
                  { IX. Trachymedusæ    { 21. Marsiporchida| Trachynema
  =Hood-jellies=  { _Hard Jelly-fish_   { 22. Phyllorchida | Geryonia
                  {                     { 23. Elasmorchida | Charybdæ
     +Medusa+     {                                        |
                  { X. Calycozoa        } 24. Podactinaria | Lucernaria
                  { _Stalked Jellies_   }                  |
                  {                                        |
                  { XI. Discomedusæ     { 25. Semæostomeæ  | Aurelia
                  { _Disc-jellies_      { 26. Rhizostomeæ  | Crambessa
       IV.        { XII. Eurystoma      } 27. Beroida      | Beroe
                  { _Wide-mouthed_      }                  |
  =Comb-jellies=  {                                        |
                  { XIII. Stenostoma    { 28. Saccata      | Cydippe
   +Ctenophora+   { _Narrow-mouthed_    { 29. Lobata       | Eucharis
                  {                     { 30. Tæniata      | Cestum

        ----------------------------------------------------------------

                 Ctenophora                      Hydromedusæ
             Tæniata   Lobata                    Rhizostomeæ
                 |        |                           |
                 |        |                           |
                 \---v----/                      Semæostomeæ
                  Saccata                        DISCOMEDUSÆ
                STENOSTOMA                            |
                     |      Trachymedusæ              |   Siphonophora
                     |            |                   |         |
                     |            |   Lucernaria      |         |
                EURYSTOMA>        |   Calycozoa       |         |
                     |            |       |           |         |
                     |            |       |           |         |
                     \-------------------------v----------------/
                                           LEPTOMEDUSÆ
                              Coralla          |
                                   Octocoralla |
                         Hexacoralla    |      |
                              |         |      |
                              |         |      |
                              \----v----/      |
                               Tetracoralla    |
                Spongiæ                 |      |
        Fibrospongiæ     Calcispongiæ   |      |
         Chalynthina   Leucones  Sycones|      |
              |           |        |    |      |
              |           |        |    |      |
              |           |        |    |      |
  Hexactinella|Geodina Dyssycus Sycurus |    Hydroida
       |      |   |       |        |    |      | Cordylophora
       |      |   |       |        |    |      |     |     Hydra
       \------v---/       \-----v--/    |      |     |       |
              |              Ascones    |      |     |       |
              |  Myxospongia    |       |      \-----v-------/
              |  Halisarcina    |       |         HYDROIDA
              |      |          |   Procorallum        |
              |      |          |       |              |
         CHALYNTHUS  |       OLYNTHUS   |              |
              |      |          |       \-----v--------/
              \------v----------/        Hydroida
                Archispongiæ                 |
                     |                       |
                     |                    Archydra
                     |                       |
                     \--------------v--------/
                                Protascus
                                 Gastræa

The second class of Sea-nettles is formed by the _Hood-jellies_ (Medusæ)
or _Polyp-jellies_ (Hydromedusæ). While most corals form stocks like
plants, and are attached to the bottom of the sea, the Hood-jellies
generally swim about freely in the form of gelatinous bells. There are,
however, numbers of them, especially the lower forms, which adhere to
the bottom of the sea, and resemble pretty little trees. The lowest and
simplest members of this class are the little fresh-water polyps (Hydra
and Cordylophora). We may look upon them as but little changed
descendants of those _Primæval polyps_ (Archydræ), from which, during
the primordial period, the whole division of the Sea-nettles originated.
Scarcely distinguishable from the Hydra are the adherent Hydroid polyps
(Campanularia, Tubularia), which produce freely swimming medusæ by
budding, and out of the eggs of these there again arise adherent polyps.
These freely swimming Hood-jellies are mostly of the form of a mushroom,
or of an umbrella, from the rim of which many long and delicate
tentacles hang. They are among the most beautiful and most interesting
inhabitants of the sea. The remarkable history of their lives, and
especially the complicated alternation of generation of polyps and
medusæ, are among the strongest proofs of the truth of the theory of
descent. For just as Medusæ still daily arise out of the Hydroids, did
the freely swimming medusa-form originally proceed, phylogenetically,
out of the adherent polyp-form. Equally important for the theory of
descent is the remarkable _division of labour_ of the individuals, which
among some of them is developed to an astonishingly high degree, more
especially in the splendid _Siphonophora_.(37) (Plate VII. Fig. 13.)

The third class of Sea-nettles—the peculiar division of Comb-jellies
(Ctenophora), probably developed out of a branch of the Hood-jellies.
The Ctenophora, which are also called Ribbed-jellies, possess a body of
the form of a cucumber, which, like the body of most Hood-jellies, is as
clear and transparent as crystal or cut glass. Comb or Ribbed-jellies
are characterized by their peculiar organs of motion, namely, by eight
rows of paddling, ciliated leaflets, which run in the form of eight ribs
from one end of the longitudinal axis (from the mouth) to the opposite
end. Those with narrow mouths (Stenostoma) probably developed later out
of those with wide mouths (Eurystoma). (Compare Plate VII. Fig. 16.)

The third tribe of the animal kingdom, the phylum of _Worms_ or
worm-like animals (Vermes, or Helminthes), contains a number of
diverging branches. Some of these numerous branches have developed into
well-marked and perfectly independent classes of Worms, but others
changed long since into the original, radical forms of the four higher
tribes of animals. Each of these four higher tribes (and likewise the
tribe of Zoophytes) we may picture to ourselves in the form of a lofty
tree, whose branches represent the different classes, orders, families,
etc. The phylum of Worms, on the other hand, we have to conceive as a
low bush or shrub, out of whose root a mass of independent branches
shoot up in different directions. From this densely branched shrub, most
of the branches of which are dead, there rise four high stems with many
branches. These are the four lofty trees just mentioned as representing
the higher phyla—the Echinoderma, Articulata, Mollusca, and Vertebrata.
These four stems are directly connected with one another at the root
only, to wit, by the common primary group of the Worm tribe.

The extraordinary difficulties which the systematic arrangement of Worms
presents, for this reason merely, are still more increased by the fact
that we do not possess any fossil remains of them. Most of the Worms had
and still have such soft bodies that they could not leave any
characteristic traces in the neptunic strata of the earth. Hence in this
case again we are entirely confined to the records of creation furnished
by ontogeny and comparative anatomy. In making then the exceedingly
difficult attempt to throw a few hypothetical rays of light upon the
obscurity of the pedigree of Worms, I must therefore expressly remark
that this sketch, like all similar attempts possesses only a provisional
value.

The numerous classes distinguished in the tribe of Worms, and which
almost every zoologist groups and defines according to his own personal
views, are, in the first place, divided into two essentially different
groups or branches, which in my Monograph of the Calcareous Sponges I
have termed Acœlomi and Cœlomati. For all the lower Worms which are
comprised in the class of Flat-worms (Platyhelminthes), (the
Gliding-worms, Sucker-worms, Tape-worms), differ very strikingly from
other Worms, in the fact that they possess neither blood nor body-cavity
(no cœlome); they are, therefore, called Acœlomi. The true cavity, or
cœlome, is completely absent in them as in all the Zoophytes; in this
important respect the two groups are directly allied. But _all other
Worms_ (like the four higher tribes of animals) possess a genuine
body-cavity and a vascular system connected with it, which is filled
with blood; hence we class them together as _Cœlomati_.

The main division of _Bloodless Worms_ (Acœlomi) contains, according to
our phylogenetic views, besides the still living Flat-worms, the unknown
and extinct primary forms of the whole tribe of Worms, which we shall
call the Primæval Worms (Archelminthes). The type of these _Primæval
Worms_, the ancient Prothelmis, may be directly derived from the Gastræa
(p. 133). Even at present the Gastrula-form—the faithful historical
portrait of the Gastræa—recurs in the ontogenesis of the most different
kinds of worms as a transient larva-form. The ciliated Gliding-worms
(Turbellaria), the primary group of the present Planary or Flat-worms
(Platyhelminthes), are the nearest akin to the Primæval Worms. The
parasitical Sucker-worms (Trematoda) arose out of the Gliding-worms,
which live freely in water, by adaptation to a parasitical mode of life;
and out of them later on—by an increasing parasitism—arose the
Tape-worms (Cestoda).

Out of a branch of the Acœlomi arose the second main division of the
Worm tribe, the Worms with blood and body-cavity (Cœlomati): of these
there are seven different classes.

The Pedigree on p. 151 shows how the obscure phylogeny of the seven
classes of Cœlomati may be supposed to stand. We shall, however, mention
these classes here quite briefly, as their relationships and derivation
are, at present, still very complicated and obscure. More numerous and
more accurate investigations of the ontogeny of the different Cœlomati
will at some future time throw light upon their phylogenesis.

The Round Worms (Nemathelminthes) which we mention as the first class of
the Cœlomati, and which are characterized by their cylindrical form,
consist principally of parasitical Worms which live in the interior of
other animals. Of human parasites, the celebrated Trichinæ, the
Maw-worms, Whip-worms, etc., for example, belong to them. The Star-worms
(Gephyrea) which live exclusively in the sea are allied to round worms,
and the comprehensive class of Ring-worms (Annelida) are allied to the
former. To the Ring-worms, whose long body is composed of a number of
segments, all alike in structure, belong the Leeches (Hirudinea),
Earth-worms (Lumbricina), and all the marine bristle-footed Worms
(Chætopoda). Nearly akin to them are the Snout-worms (Rhynchocœla), and
the small microscopic Wheel-worms (Rotifera). The unknown, extinct,
primary forms of the tribe of Sea-stars (Echinoderma), and of the tribe
of the articulated animals (Arthropoda), were nearest akin to the
Ring-worms. On the other hand, we must probably look for the primary
forms of the great tribe of Molluscs in extinct Worms, which were very
closely related to the Moss-polyps (Bryozoa) of the present day; and for
the primary forms of the Vertebrata in the unknown Cœlomati, whose
nearest kin of the present day are the Sea-sacs, especially the Ascidia.

SYSTEMATIC SURVEY

_Of the 8 Classes and 22 Orders of the Worm Tribe._

(Compare Gen. Morph. ii. Plate V. pp. 75-77.)

  ------------------+-------------------------+----------------------+------------------
  _Classes_         |                         |  _Systematic_        |
  _of the_          |  _Orders of the_        |  _Name of the_       | _Name of a Genus_
  _Worm Tribe._     |   _Worm Tribe._         | _Orders of Worms_.   |  _as example._
                    |                         |                      |
  ------------------+-------------------------+----------------------+------------------
  1. _Flat_          {  1. Primæval worms     |  1. Archelminthes    | Prothelmis
  _Worms_            {  2. Gliding-worms      |  2. Turbellaria      | Planaria
  Platyhel-          {  3. Sucker-worms       |  3. Trematoda        | Distoma
  minthes            {  4. Tape-worms         |  4. Cestoda          | Tænia
                                              |                      |
  2. _Round_         {  5. Arrow-worms        |  5. Chætognatha      | Sagitta
  _Worms_            {  6. Thread-worms       |  6. Nematoda         | Trichina
  Nemathel-          {  7. Hook-headed        |  7. Acanthocephala   | Echinorhynchus
  minthes            {      worms             |                      |
                                              |                      |
  3. _Moss_          }  8. Horse-shoe-lipped  |  8. Lophopoda        | Alcyonella
  _Polyps_           }  9. Circle-lipped      |  9. Stelmopoda       | Retepora
  Bryozoa            }                        |                      |
                                              |                      |
  4. _Sea-sacs_      { 10. Sea-squirts        | 10. Ascidia          | Phallusia
  Tunicata           { 11. Sea-barrels        | 11. Thaliacea        | Salpa
                                              |                      |
  5. _Proboscideans_ } 12. Tongue-worms       | 12. Enteropneusta    | Balanoglossus
  Rhynchocœla        } 13. Cord-worms         | 13. Nemertina        | Borlasia
                                              |                      |
                     { 14. Star-worms without | 14. Sipunculida      | Sipunculus
  6. _Star-Worms_    {       bristles         |                      |
  Gephyrea           { 15. Star-worms with    | 15. Echiurida        | Echiurus
                     {       bristles         |                      |
                                              |                      |
  7. _Wheel_         }                        |                      |
  _Animalcule_       } 16. Wheel-worms        | 16. Rotatoria        | Hydatina
  Rotifera           }                        |                      |
                                              |                      |
                     { 17. Bear-worms         | 17. Arctisca         | Macrobiotus
                     { 18. Worms with claws   | 18. Onychophora      | Peripatus
  8. _Ring_          { 19. Leeches            | 19. Hirudinea        | Hirudo
  _Worms_            { 20. Land-worms         | 20. Drilomorpha      | Lumbricus
  Annelida           { 21. Mailed worms       | 21. Phracthelminthes | Crossopodia
                     { 22. Bristle-footed     | 22. Chætopoda        | Aphrodite
                     {      worms             |                      |

        ----------------------------------------------------------------

                       Chætopoda
                           |          Drilomorpha
                           |               |
                           \-------v-------/
                  Phracthelminthes |
                          |        |
                 Echiurida|        |   Hirudinea
        Sipunculida  |    |        |       |
            |        |    |        |       |   Onychophora
            \----v---/    |        |       |        |
               +Gephyrea+ |        |       |        |  Arctisca
                     |    |        |       |        |     |
                     |    |        |       |        \--v-—/
      Chætognatha    |    \--v-----/       |           |
           |         |       |             \-----v-----/
  Nematoda |         |       |                   |
     |     |         |       \---------v---------/
     \--v--/         |              +Annelida+
        |  Acantho-  |                   |
        |   cephala  | Stelmopoda        |
        |     |      |     |             |              Enteropneusta
        \--v--/      |     |             |                   |
   +Nemathelminthes+ | Lophopoda         |         Ascidia   |
           |         |  +Bryozoa+        |  Thalicea |       |  Nemertina
           |         |     |             |    |      |       |     |
           |         |     | +Rotifera+  |    |      |       |     |
           |         |     |    |        |    |      |       \---v-/
           |         |     |    |        |    |      |    +Rhynchocœla+
           |         |     |    |        |    \--v---/           |
           |         |     |    \----v---/    +Tunicata+         |
           |         |     |         |           |               |
           \-----------------------v-----------------------------/
                  +Cœlomati+ (_worms with body-cavity_)
             Cestoda               |
                 |                 |
                 |                 |
             Trematoda             |
                 |                 |
                 |                 |
           Turbellaria             |
         +Platyhelminthes+         |
                 |                 |
                 |                 |
                 \-------v---------/
             +Acœlomi+ (_worms without body-cavity_)
                   Archelminthes
                    +Prothelmis+
                      +Gastræa+

The class of Sea-sacs (Tunicata) is one of the most remarkable among
Worms. They all live in the ocean, where some of the Ascidiæ adhere to
the bottom, while others (the sea-barrels, or Thaliacea) swim about
freely. In all of them the non-jointed body has the form of a simple
barrel-shaped sack, which is surrounded by a thick cartilaginous mantle.
This mantle consists of the same non-nitrogenous combination of carbon,
which, under the name of cellulose, plays an important part in the
Vegetable Kingdom, and forms the largest portion of vegetable cellular
membranes, and consequently also the greater part of wood. The
barrel-shaped body generally possesses no external appendages. No one
would recognise in them a trace of relationship to the highly
differentiated vertebrate animals. And yet this can no longer be
doubted, since Kowalewsky’s investigations, which in the year 1867
suddenly threw an exceedingly surprising and unmistakable light upon
them. From these investigations it has become clear that the individual
development of the adherent simple Ascidian Phallusia agrees in most
points with that of the lowest vertebrate animal, namely, the Lancelet
(Amphioxus lanceolatus). The early stages of the Ascidia possess the
beginnings of the _spinal marrow_ and the _spinal column_ (chorda
dorsalis) lying beneath it, which are the two most essential and most
characteristic organs of the vertebrate animal. Accordingly, of all
invertebrate animals known to us, the _Tunicates are without doubt the
nearest blood relations of the Vertebrates_, and must be considered as
the nearest relations of those Worms out of which the vertebrate tribe
has developed. (Compare Plates XII. and XIII.)

While thus different branches of the Cœlomatous group of the Worms
furnish us with several genealogical links leading to the four higher
tribes of animals, and give us important phylogenetic indications of
their origin, the lower group of Acœlomi, on the other hand, show close
relationships to the Zoophytes, and to the Primæval animals. The great
phylogenetic interest of the Worm tribe rests upon this peculiar
intermediate position.

CHAPTER XIX.

PEDIGREE AND HISTORY OF THE ANIMAL KINGDOM.

II. MOLLUSCA, STAR-FISHES, AND ARTICULATED ANIMALS.

  Tribe of Molluscs.—Four Classes of Molluscs: Lamp-shells
  (Spirobranchia); Mussels (Lamellibranchia); Snails (Cochlides);
  Cuttle-fish (Cephalopoda).—Tribe of Star-fishes, or
  Echinoderma.—Their Derivation from Ringed Worms (Mailed Worms,
  or Phracthelminthes).—The Alternation of Generation in the
  Echinoderma.—Four Classes of Star-fish: Sea-stars (Asteridea);
  Sea-lilies (Crinoidea); Sea-urchins (Echinidea); Sea-cucumbers
  (Holothuridea).—Tribe of Articulated Animals, or
  Arthropoda.—Four Classes of Articulated Animals: Branchiata, or
  Crustacea, breathing through gills; Jointed Crabs; Mailed
  Crabs; Articulata Tracheata, breathing through Air
  Tubes.—Spiders (Long Spiders, Round
  Spiders).—Myriopods.—Insects.—Chewing and Sucking
  Insects.—Pedigree and History of the Eight Orders of Insects.

The great natural main groups of the animal kingdom, which we have
distinguished as TRIBES, or PHYLA (“types” according to Bär and Cuvier),
are not all of equal systematic importance for our phylogeny or history
of the pedigree of the living world. They can neither be classed in a
single series of stages, one above another, nor be considered as
entirely independent stems, nor as equal branches of a single
family-tree. It seems rather (as we saw in the last chapter) that the
tribe of Protozoa, the so-called primæval animals, is the common radical
group of the whole animal kingdom. Out of the Gastræada—which we class
among the Protozoa—the Zoophytes and the Worms have developed, as two
diverging branches. We must now in turn look upon the varied and
much-branching tribe of Worms as the common primary group, out of which
(from perfectly distinct branches) arose the remaining tribes, the four
higher phyla of the animal kingdom. (Compare the Pedigree, p. 133.)

Let us now take a genealogical look at these four higher tribes of
animals, and try whether we cannot make out the most important outlines
of their pedigree. Even should this attempt prove defective and
imperfect, we shall at all events have made a beginning, and paved the
road for subsequent and more satisfactory attempts.

It does not matter in what succession we take up the examination of the
four higher tribes. For these four phyla have no close relationship
whatever among one another, but have grown out from entirely distinct
branches of the group of Worms (p. 133). We may consider the tribe of
Molluscs as the most imperfect and the lowest in point of morphological
development. We nowhere meet among them with the characteristic
articulation or segmented formation of the body, which distinguishes
even the Ring-worms, and which in the other three higher tribes—the
Echinoderma, Articulata, and Vertebrata—is most essentially connected
with the high development of their forms, their differentiation, and
perfection. The body in all Molluscs—in mussels, snails, etc.—is a
simple non-jointed sack, in the cavity of which lie the intestines. The
nervous system consists not of a cord but of several distinct (generally
three) pairs of knots loosely connected with one another. For these and
many other anatomical reasons, I consider the tribe of Molluscs (in
spite of the high physiological development of its most perfect forms)
to be morphologically the lowest among the four higher tribes of
animals.

Whilst, for reasons already given, we exclude the Moss-polyps, and
Tunicates—which have hitherto been generally classed with the tribe of
Molluscs—we retain as genuine Molluscs the following four classes:
Lamp-shells, Mussels, Snails, and Cuttles. The two lower classes of
Molluscs, the Lamp-shells and Mussels, possess neither head nor teeth,
and they can therefore be comprised under one main class, or branch, as
headless animals (Acephala), or toothless animals (Anodontoda). This
branch is also frequently called that of the clam-shells (Conchifera, or
Bivalvia), because all its members possess a two-valved calcareous
shell. In contrast to these the two higher classes of Molluscs, the
snails and cuttles, may be represented as a second branch with the name
of Head-bearers (Cephalophora), or Tooth-bearers (Odontophora), because
both head and teeth are developed in them.

The soft, sack-shaped body in most Molluscs is protected by a calcareous
shell or house, which in the Acephala (lamp-shells and mussels) consists
of two valves, but in the Cephalophora (snails and cuttles) is generally
a spiral tube (the so-called snail’s house). Although these hard
skeletons are found in large quantities in a petrified state in all the
neptunic strata, yet they tell us but little of the historical
development of the tribe, which must have taken place for the most part
in the primordial period. Even in the Silurian strata we find fossil
remains of all the four classes of Molluscs, one beside the other, and
this, conjointly with much other evidence, distinctly proves that the
tribe of Molluscs had then obtained a strong development, when the
higher tribes, especially the Articulates and Vertebrates, had scarcely
got beyond the beginning of their historical development. In subsequent
periods, especially in the primary and secondary periods, these higher
tribes increased in importance more and more at the expense of Molluscs
and Worms, which were no match for them in the struggle for life, and
accordingly decreased in number. The still living Molluscs and Worms
must be considered as only a proportionately small remnant of the vast
molluscan fauna, which greatly predominated in the primordial and
primary periods over the other tribes. (Compare Plate VI. and
explanation in the Appendix.)

No tribe of animals shows more distinctly than do the Molluscs, how very
different the value of fossils is in geology and in phylogeny. In
geology the different species of the fossil shells of Molluscs are of
the greatest importance because they serve as excellent marks whereby to
characterize the different groups of strata, and to fix their relative
ages. As far as relates to the genealogy of Molluscs, however, they are
of very little value, because, on the one hand, the shells are parts of
quite subordinate morphological importance, and because the actual
development of the tribe belongs to the earlier primordial period, from
which no distinct fossils have been preserved. If therefore we wish to
construct the pedigree of Molluscs, we are mainly dependent upon the
records of ontogeny and comparative anatomy from which we obtain
something like the following result. (Gen. Morph. ii. Plate VI. pp.
102-116.)

The lowest stage of the four classes of genuine Molluscs known to us, is
occupied by the Lamp-shells or Spiral-gills (Spirobranchia), frequently
but inappropriately called Arm-footers (Brachiopoda), which have become
attached to the bottom of the sea. There now exist but few forms of
this class; for instance, some species of Lingula, Terebratula, and
others akin to them, which are but feeble remnants of the great variety
of forms which represented the Lamp-shells in earlier periods of the
earth’s history. In the Silurian period they constituted the principal
portion of the whole Mollusc tribe. From the agreement which, in many
respects, their early stage of development presents with the Moss
animals, it has been concluded that they have developed out of Worms,
which were nearly related to this class. Of the two sub-classes of
Lamp-shells, the Hinge-less (Ecardines) must be looked upon as the lower
and more imperfect, the Hinged (Testicardines) as the higher and more
fully developed group.

The anatomical difference between the Lamp-shells and the three other
classes of Molluscs is so considerable that the latter may be
distinguished from the former by the name of Otocardia. All the
Otocardia have a heart with chamber (ventricle) and ante-chamber
(auricle), whereas Lamp-shells do not possess the ante-chamber.
Moreover, the central nervous system is developed only in the former
(and not in the latter) in the shape of a complete pharyngeal ring.
Hence the four classes of Molluscs may be grouped in the following
manner:—

                     {     1. Lamp-shells     }     I. Haplocardia
  I.  Molluscs       {     (Spirobranchia).   }     (with simple heart).
  without head.      {
  _Acephala._        {     2. Mussels         }
                     {     (Lamellibranchia). }     II. Otocardia
                                              }     (with chamber
  II.  Molluscs      {     3. Snails          }     and ante-chamber
  with head.         {     (Cochlides).       }     to the heart).
  _Cephalophora._    {     4. Cuttles         }
                     {     (Cephalopoda).     }

The result of these structural dispositions for the history of the
pedigree of Molluscs, which is confirmed by palæontology, is that
Lamp-shells stand much nearer to the primæval root of the whole tribe of
Molluscs than do the Otocardia. Probably Mussels and Snails developed as
two diverging branches out of Molluscs, which were nearly akin to the
Lamp-shells.

Mussels, or Plate-gills (Lamellibranchia), possess a bivalved shell like
the Lamp-shells. In the latter, one of the two valves covers the back,
the other the belly of the animal; whereas in Mussels the two valves lie
symmetrically on the right and left side of the body. Most Mussels live
in the sea, only a few in fresh water. The class is divided into two
sub-classes, Asiphonia and Siphonida, of which the latter were developed
at a later period out of the former. Among the Asiphonia are Oysters,
mother-of-pearl Shells, and fresh water Mussels; among the Siphonida,
which are characterized by a respiratory tube, are the Venus-shells,
Razor-shells, and Burrowing Clams. The higher Molluscs seem to have
developed at a later period out of those without head and teeth; they
are distinguished from the latter by the distinct formation of the head,
and more especially by a peculiar kind of tooth apparatus. Their tongue
presents a curious plate, armed with a great number of teeth. In our
common Vineyard Snail (Helix pomatia) the number of teeth amount to
21,000, and in the large Garden Slug (Limax maximus) to 26,800.

SYSTEMATIC SURVEY

_Of the 4 Classes, 8 Sub-classes, and 21 Orders of Molluscs._

  ------------------+-----------------------+------------------------+-------------------
    _Classes of_    |    _Sub-classes of_   |    _Orders of_         |  _Systematic Name_
    _Molluscs._     |      _Molluscs._      |    _Molluscs._         |   _of the Orders._
  ------------------+-----------------------+------------------------+-------------------
          I. _Molluscs without head or teeth_: ACEPHALA _or_ ANODONTODA.
  ---------------------------------------------------------------------------------------
      I.            {   I. Ecardines        { 1. Stalked              1. Lingulida
  =Lamp-shells=     {      _Hinge-less_     { 2. Flattened            2. Craniada
                    {
  +Spirobranchia+   {
       or           {
  +Brachiopoda+     {  II. Testicardines    { 3. Fleshy armed         3. Sarcobrachia
                    {     _Hinge-less_      { 4. Calcareous-armed     4. Sclerobrachia

      II.           { III. Asiphonia        { 5. One-muscled          5. Monomya
    =Mussels=       { _Mussels without_     { 6. Uneven-muscled       6. Heteromya
      or            { _respiratory tubes_   { 7. Even-muscled         7. Isomya
  =Plate-gills=     {
                    {
  +Lamellibranchia+ {  IV. Siphonida        { 8. Round-mantled        8. Integripallia
        or          {  _Mussels with_       { 9. Ray-mantled          9. Sinupalliata
  +Phyllobranchia+  { _respiratory tubes_   { 10. Tube-mussels       10. Inclusa

  ---------------------------------------------------------------------------------------
          II. _Molluscs with head and teeth_: CEPHALOPHORA _or_ ODONTOPHORA.
  ---------------------------------------------------------------------------------------

                    {   V. Stump-headed     { 11. Tube-snails        11. Scaphopoda
      III.          {      _Perocephala_    { 12. Butterfly-snails   12. Pteropoda
     =Snails=       {
                    {                       { 13. With hind gills    13. Opisthobranchia
   +Cochlides+      {                       { 14. With fore gills    14. Prosobranchia
       or           {   VI. Large-headed    { 15. Swimming-snails    15. Heteropoda
  +Gasteropoda+     {      _Delocephala_    { 16. Beetle-snails      16. Chitonoida
                    {                       { 17. Snails with lungs  17. Pulmonata

                       VII. Chamber-Poulps  { 18. Pearl boats        18. Nautilida
      IV.                  with four gills  { 19. Ammon’s horns      19. Ammonitida
    =Cuttles=       }     _Tetrabranchia_   {
      or            }
    =Poulps=        } VIII. Ink-Poulps with { 20. Ten-armed          20. Decabrachiones
                    }       two gills       {
  +Cephalopoda+     }       _Dibranchia_    { 21. Eight-armed        21. Octobrachiones

        ----------------------------------------------------------------

                                                      Dibranchia
                         Heteropoda                        |
                             |       _Prosobranchia_       |
  _Pulmonata_                |              |         Tetrabranchia
      |      Lipobranchia    |              |         Cephalopoda
      |            |         |              |    (=Cuttles= or =Poulps=)
      |     Gymnobranchia    |              |               |
      |            |         \-------v------/               |
      |            |                 |                      |
      |     Pleurobranchia           |       Chitonides     |
      |    _Opisthobranchia_         |            |         |
      |            |                 |            |         |
      \---------------------------------v---------/         |
                                        |                   |
                                   _Delocephala_            |
                                        |                   |
                                        |                   |
                                        \--------v----------/
                                                  |       _Pteropoda_
                        Inclusa                   |            |
                           |                      |            |
                           |                      \------v-----/
                       Sinupalliata                      |
                           |                  Scaphopoda |
                           |                      |      |
                     Integripalliata              \---v--/
  Sclerobrachia       _Siphoniata_                    |
        |                   |                   _Perocephala_
        |                   |                     Cochlides
        |                   |                     (=Snails=)
   Sarcobrachia        _Asiphonia_                    |
  _Testicardines_     Lamellibranchia                 |
        |                (=Mussels=)                  |
        |                    |                        |
   _Ecardines_               \----------v-------------/
  Spirobranchia                         |
   (=Lamp-shells=)                     Otocardia
        |            (Molluscs with chamber and ante-chamber
        |                          to the heart)
        |                                |
        |                                |
        \-----------------v--------------/
              Promollusca (Primæval Molluscs)
                Molluscs with simple heart
                       (Worms)
                       Gastræa

We distinguish two sub-classes among the Snails (Cochlides, or
Gasteropoda), namely, the Stump-headed and the Large-headed Snails. The
Stump-headed Snails (Perocephala) are very closely allied to Mussels
(through the Tooth-shells), and also to the Cuttle-fish (through the
Butterfly-snails). The more highly developed Snails, with large heads
(Delocephala), can be divided into Snails with gills (Branchiata) and
Snails with lungs (Pulmonata). Among the latter are the Land-snails, the
only Molluscs which have left the water and become habituated to a life
on land. The great majority of Snails live in the sea, only a few live
in fresh water. Some River-snails in the tropics (the Ampullaria) are
amphibious, living sometimes on land, sometimes in water, and at one
time they breathe through gills, at another through lungs. They have
both kinds of respiratory organs, like the Mud-fish and Gilled Newts
among the Vertebrata.

The fourth and last class, and at the same time the most highly
developed class of Molluscs, is that of the Cuttles, or Poulps, also
called Cephalopoda (foot attached to the head). They all live in the
sea, and are distinguished from Snails by eight, ten, or more long arms,
which surround the mouth in a circle. The Cuttles existing in our recent
oceans—the Sepia, Calamary, Argonaut, and Pearly Nautilus—are, like the
few Spiral-gill Lamp-shells of the present time, but a poor remnant of
the host which represents this class in the oceans of the primordial,
primary, and secondary periods. The numerous fossil “Ammon’s horns”
(Ammonites), “pearl boats” (Nautilus), and “thunderbolts” (Belemnites)
are evidences of the long since extinct splendour of the tribe. The
Poulps, or Cuttles, have probably developed out of a low branch of the
snail class, out of the Butterfly-snails (Pteropoda) or kindred forms.

The different sub-classes and orders, distinguished in the four classes
of Molluscs, whose systematic succession is given on the Table (p.
160), furnish various proofs of the validity of the law of progress by
their historical development and by the systematic development
corresponding to it. As however these subordinate groups of Molluscs are
in themselves of no further special interest, I must refer to the sketch
of their pedigree on p. 161, and to the detailed pedigree of Molluscs
which I have given in my General Morphology, and I shall now at once
turn to the consideration of the tribe of Star-fishes.

The Star-fishes (Echinoderma, or Estrellæ) among which are the four
classes of Sea-stars, Sea-lilies, Sea-urchins, and Sea-cucumbers are one
of the most interesting divisions of the animal kingdom, and yet we know
less about them than about any. They all live in the sea. Every one who
has been at the sea shore must have seen at least two of their forms,
the Sea-stars and the Sea-urchins. The tribe of Star-fishes must be
considered as a completely independent tribe of the animal kingdom on
account of its very peculiar organization, and must be carefully
distinguished from the Animal-plants—Zoophytes, or Cœlenterata, with
which it is still frequently but erroneously classed under the name
Radiata (as for example, by Agassiz, who even to this day defends this
error of Cuvier’s, together with many others).

All Echinoderma are characterized, and at the same time distinguished
from all other animals, by a very remarkable apparatus for locomotion,
which consists of a complicated system of canals or tubes, filled with
sea water from without. The sea water in these aqueducts is moved partly
by the strokes of the cilia, or vibratile hairs lining their walls, and
partly by the contractions of the muscular walls of the tubes
themselves, which resemble india-rubber bags. The water is pressed from
the tubes into a number of little hollow feet, which thereby become
widely distended, and are then employed for walking and suction. The
Sea-stars are moreover characterized by a peculiar calcareous formation
in the skin, which in most cases forms a firm, well-closed coat of mail,
composed of a number of plates. In almost all Echinoderma the body
consists of five radii (counterparts, or antimera) standing round the
main axis of the body, where they meet. It is only in some species of
Sea-stars that the number of these radii amount to more than five—to
6-9, 10-12, or even to 20-40; and in this case the number of radii is
generally not constant, but varies in different individuals of one
species.

The historical development and the pedigree of the Echinoderma are
completely revealed to us by their numerous and, in most cases,
excellently preserved fossil remains, by their very remarkable
individual developmental history, and by their interesting comparative
anatomy; this is the case with no other tribe of animals, even the
Vertebrata themselves are not to be excepted. By a critical use of those
three archives, and by a careful comparison of the results derived from
their study, we obtain the following genealogy of the Star-fishes, which
I have already published in my General Morphology (vol. ii. Plate IV.
pp. 62-77.)

The most ancient and original group of the Star-fishes, the primary form
of the whole phylum, consists of the class of the true Sea-stars
(Asterida). This is established by numerous and important arguments in
anatomy and the history of development, but above all by the irregular
and varying number of the radii, or antimera, which in all other
Echinoderma is limited, without exception, to five. Every Star-fish
consists of a central, small, body-disc, all round the circumference of
which are attached five or several long articulated arms. _Each arm of
the Star-fish essentially corresponds in its organisation with an
articulated worm_ of the class of Ring-worms, or Annelida (p. 149). I
therefore consider the Star-fish as a genuine _stock or cormus of five
or more articulated worms_, which have arisen by the star-wise growth of
a number of buds out of a central mother-worm. The connected members,
thus grouped like the rays of a star, have inherited from the
mother-worm the common opening of the mouth, and the common digestive
cavity (stomach) lying in the central body-disc. The end by which they
have grown together, and which fuses in the common central disc,
probably corresponds to the posterior end of the original independent
worms.

In exactly the same way several individuals of certain kinds of worms
are united so as to form a star-like cormus. This is the case in the
_Botryllidæ_, compound Ascidians, belonging to the class of the
Tunicata. Here also the posterior ends of the individual worms have
grown together, and have formed a common outlet for discharges, a
central cloaca; whereas at the anterior end each worm still possesses
its own mouth. In Star-fishes the original mouths have probably become
closed in the course of the historical development of the cormus, or
colony, whereas the cloaca has developed into a common mouth for the
whole cormus.

SYSTEMATIC SURVEY

_Of the 4 Classes, 9 Sub-classes, and 20 Orders of Star-fishes._

(Compare Gen. Morph. II. Plate IV. pp. 62-67.)

  ----------------+-------------------------+---------------------------+------------------
  _Classes of the | _Sub-classes of the     |    _Orders of the         | _Systematic Name
  Star-fishes._   | Star-fishes._           |      Star-fishes._        |  of the Orders._
  ----------------+-------------------------+---------------------------+------------------
                   { I.                      { 1. Primary Stars         | 1. Tecastra
                   { Sea Stars with radiated { 2. Articulated Stars     | 2. Colastra
  I.               { stomach                 { 3. Brisinga Stars        | 3. Brisingastra
  =Sea Stars=      { _Actinogastra_                                     |
  +Asterida+       {
                   { II.                     { 4. Serpent Stars         | 4. Ophiastra
                   { Sea Stars with disc-    { 5. Tree Stars            | 5. Phytastra
                   { shaped stomach          { 6. Lily Stars            | 6. Crinastra
                   { _Discogastra_           {                          |
                   { III.                    { 7. Plated Lilies with    | 7. Phatnocrinida
                   { Lilies with arms        {      arms                |
                   { _Brachiata_             { 8. Articulated Lilies    | 8. Colocrinida
                   {                         {      with arms           |
  II.              { IV.                     { 9. Regularly budding     | 9. Pentremitida
  =Sea Lilies=     { Lilies with buds        {      Lilies              |
  +Crinoida+       { _Blastoidea_            { 10. Lilies budding on    | 10. Eleutherocrina
                   {                         {      two sides           |
                   { V.                      { 11. Bladder Lilies       | 11. Agelacrinida
                   { Bladder Lilies          {       without stalks     |
                   { _Cystidea_              { 12. Bladder Lilies       | 12. Sphæronitida
                                             {       with stalks        |
                   { VI.                     { 13. Palechinida with     | 13. Melonitida
                   { Older Sea Urchins       {       more than 10       |
                   { (with more than         {       rows of ambulacral |
                   { 20 rows of plates)      {        plates            |
  III.             { _Palechinida_           { 14. Palechinida with     | 14. Eocidaria
  =Sea Urchins=    {                         {       10 rows of         |
  +Echinida+       { VII.                    {       ambulacral plates  |
                   { More recent Sea         { 15. Autechinida with     | 15. Desmosticha
                   { Urchins (with 20        {      band-like ambulacra |
                   { rows of plates)         { 16. Autechnidia with     | 16. Petalosticha
                   { _Autechinida_           {      leaf-like ambulacra |
                   { VIII.                   { 17. Eupodia with scuti-  | 17. Aspidochirota
                   { Sea Cucumbers           {       form tentacles     |
                   { with aquatic feet       { 18. Eupodia with         | 18. Dendrochirota
  IV.              { _Eupodia_               {      branching tentacles |
  =Sea Cucumbers=  {                         {                          |
                   { IX.                     {                          |
  +Holothuriæ+     { Sea Cucumbers           { 19. Apodia with water-   | 19. Liodermatida
                   { without aquatic         {       lungs              |
                   { feet                    { 20. Apodia without       | 20. Synaptida
                   { _Apodia_                {       water-lungs        |

             ----------------------------------------------------------------

                                                                    Clypeastridæ
                                                      Spatangidæ         |
                                                           |             |
                                              Dysasteridæ  |             |
                                                   |       |             |
                             Aspidochirota         |       |             |
                                   |               |       \------v------/
         Synaptida                 |               |              |
             |                     |               |         Cassidulidæ
             |                     |               |        +Petalosticha+
             |                     |               |              |
             |                     |               |              |
        Liodermatida               |               |              |
          +Apodia+                 |               \------v-------/     Echinonidæ
             |                     |                      |                 |
             |                     |                 Galeritidæ             |
             |                     |                      |   Echinometridæ |
             \---------v-----------/                      |         |       |
                       |                                  \------v---------/
                 Dendrochirota                                   |
                   +Eupodia+                                Latistellæ
                  +Holothuriæ+            Salenidæ               |
                \------v------/              |                   |
                       |                     |                   |
                       |                     \------v------------/
                       |                            |
                       |                      Angustistellæ
                       |                      +Desmosticha+
                       |                      +Autechinida+
                       |                            |
                       |                            |           Colocrinæ
                       |                            |               |
                       |                            | Sphæronitidæ  |
    Phytastra          |                            |       |       |
        |              |                            |       |       |
        |              |                       Eocidaridæ   |       |    Eleutherocrina
        |              |                            |       |       |           |
        |              |                            |       |       |           |
    Ophiastra          |                            |   Agelacrinæ  |           |
  +Discogastra+        |                            |   +Cystidea+  |           |
        |              |                            |       |       |           |
        |              |                            |       |       |           |
        |              |                        Melonitida  |   Phatnocrinæ     |
   Brisingastra        |                        Palechinida |   +Brahiata+   Pentremitida
        |              |                        +Echinida+  |       |        +Blastoidea+
        |              |                            |       |       |           |
        |              |                            |       |       |           |
     Colastra          \----------v-----------------/       \-------v-----------/
        |                         |                             +Brachiata+
        |                         |                              Crinoida
        |                         |                              Crinastra
        |                         |                                  |
        \-------------------------v----------------------------------/
                               Tocastra
                            +Actinogastra+
                              +Asterida+
                             Phracthelminthes
                               Cœlomati
                                Gastræa

Hence the Star-fishes would be compound stocks of worms which, by the
radial formation of buds, have developed out of true articulated worms,
or Annelids. This hypothesis is most strongly supported by the
comparative anatomy, and by the ontogeny of some Star-fishes (Colastra),
and of segmented worms. The many-jointed Ring-worms (Annelida) in their
inner structure are closely allied to the individual arms or radii of
the Star-fishes, that is to the original single worms, which each arm
represents. Each of the five worms of the Star-fish is a chain composed
of a great number of equi-formal members, or metamera, lying one behind
the other, like every segmented Worm, and every Arthropod. As in the
latter a central nervous cord, the ventral nerve cord runs along the
central line of the ventral wall of each segment. On each metameron
there is a pair of non-jointed feet, and besides these, in most cases,
one or more hard thorns or bristles similar to those of many Ring-worms.
A detached arm of a Star-fish can lead an independent life, and can
then, by the radially-directed growth of buds at one end, again become a
complete star.

The most important proofs, however, of the truth of my hypothesis are
furnished by the ontogeny or the individual development of the
Echinoderma. The most remarkable facts of this ontogeny were first
discovered in the year 1848 by the great zoologist, Johannes Müller of
Berlin. Some of its most important stages are represented on Plates
VIII. and IX. (Compare their explanation in the Appendix.) Fig. _A_ on
Plate IX. shows us a common Sea-star (Uraster), Fig. _B_, a Sea-lily
(Comatula), Fig. _C_, a Sea-urchin (Echinus), and Fig. _D_, a
Sea-cucumber (Synapta). In spite of the extraordinary difference of form
manifested by these four representatives of the different classes of
Star-fishes, yet the beginning of their development is identical in all
cases. Out of the egg an animal-form develops which is utterly
different from the fully developed Star-fish, but very like the ciliated
larvæ of certain segmented Worms (Star-worms and Ring-worms). This
peculiar animal-form is generally called the “larva,” but more correctly
the “nurse” of these Star-fish. It is very small and transparent, swims
about by means of a fringe of cilia, and is always composed of two equal
symmetrical halves or sides. The fully grown Echinoderm, however—which
is frequently more than a hundred times larger, and quite opaque—creeps
at the bottom of the sea, and is always composed of at least five
co-ordinate pieces, or antimera, in the form of radii. Plate VIII. shows
the development of the “nurses” of the four Echinoderms represented on
Plate IX.

The fully developed Echinoderm arises by a very remarkable process of
budding in the interior of the “nurse,” of which it retains little more
than the stomach. The nurse, erroneously called the “larva,” of the
Echinoderm, must accordingly be regarded as a solitary worm, which by
internal budding produces a second generation, in the form of a stock of
star-shaped and connected worms. The whole of this process is a genuine
alternation of generations, or metagenesis, not a “metamorphosis,” as is
generally though erroneously stated. A similar alternation of
generations also occurs in many other worms, especially in some star
worms (Sipunculidæ), and cord worms (Nemertinæ). Now if, bearing in mind
the fundamental law of biogeny, we refer the ontogeny of Echinoderma to
their phylogeny, then the whole historical development of the
Star-fishes suddenly becomes clear and intelligible to us, whereas
without this hypothesis it remains an insoluble mystery. (Compare Gen.
Morph. ii. pp. 95-99.)

Besides the reasons mentioned, there are many other facts (principally
from the comparative anatomy of Echinoderma) which most distinctly prove
the correctness of my hypothesis. I established this hypothesis in 1866,
without having any idea that _fossil articulated worms_ still existed,
apparently answering to the hypothetical primary forms. Such have in the
mean time, however, really been discovered. In a treatise “On the
Equivalent of the North American Taconic Schist in Germany,”[3] Geinitz
and Liebe, in 1867, have described a number of articulated Silurian
worms, which completely confirm my suppositions. Numbers of these very
remarkable worms are found in an excellent state of preservation in the
slates of Würzbach, in the upper districts of Reusz. They are of the
same structure as the articulated arm of a Star-fish, and evidently
possessed a hard coat of mail, a much denser, more solid cutaneous
skeleton than other worms in general. The number of body-segments, or
metamera, is very considerable, so that the worms, although no more than
a quarter or half an inch in breadth, attained a length of from two to
three feet. The excellently preserved impressions, especially those of
the Phyllodocites thuringiacus and Crossopodia Henrici, are so like the
arms of many Star-fish (Colastra) that their true blood relationship
seems very probable. This primæval group of worms, which are most
probably the ancestors of Star-fish, I call Mailed worms
(Phracthelminthes, p. 150.)

[Illustration: Pl. viii.

STAR FISHES. FIRST GENERATION. WORM PERSON.]

[Illustration: Pl. ix.

STAR FISHES. SECOND GENERATION. WORM STOCK.]

The three other classes of Echinoderma evidently arose at a later period
out of the class of Sea-stars which have most faithfully retained the
original form of the stellate colony of worms. The Sea-lilies, or
Crinoida, differ least from them, but having given up the free, slow
motion possessed by other Sea-stars, they have become adherent to rocks,
etc., and form for themselves a long stalk. Some Encrinites, however
(for example, the Comatulæ, Fig. _B_, on Plates VIII. and IX.),
afterwards detach themselves from their stalk. The original worm
individuals in the Crinoida are indeed no longer preserved in the same
independent condition as in the case of the common star-fish; but they
nevertheless always possess articulated arms extending from a common
central disc. Hence we may unite the Sea-lilies and Sea-stars into a
main-class, or branch, characterized as possessing articulated arms
(Colobrachia).

In the other two classes of Echinoderma, the Sea-urchins and
Sea-cucumbers, the articulated arms are no longer present as independent
parts, but, by the increased centralization of the stock, have
completely fused so as to form a common, inflated, central disc, which
now looks like a simple box or capsule without arms. The original stock
of five individuals has apparently degenerated to the form-value of a
simple individual, a single person. Hence we may represent these two
classes as a branch characterized as being without arms (Lipobrachia),
equivalent to those which possess articulated arms. The first of these
two classes, that of Sea-urchins (Echinida) takes its name from the
numerous and frequently very large thorns which cover the hard shell,
which is itself artistically built up of calcareous plates. (Fig. _C_,
Plates VIII. and IX.) The fundamental form of the shell itself is a
pentagonal pyramid. The Sea-urchins probably developed directly out of
the group of Sea-stars. The different classes and orders of marine
lilies and stars which are given in the following table, illustrate the
laws of progress and differentiation in a striking manner. In each
succeeding period of the earth’s history we see the individual classes
continually increasing in variety and perfection. (Gen. Morph. ii. Plate
IV.)

The history of three of these classes of Star-fish is very minutely
recorded by numerous and excellently preserved fossils, but on the other
hand, we know almost nothing of the historical development of the fourth
class, that of the Sea-cucumbers (Holothuriæ). These curious
sausage-shaped Star-fish manifest externally a deceptive similarity to
worms. (Fig. _D_, Plates VIII. and IX.) The skeletal structures in their
skin are very imperfect, and hence no distinct remains of their
elongated, cylindrical, worm-like body could be preserved in a fossil
state. However, from the comparative anatomy of the Holothuriæ, we can
infer that they have arisen, by the softening of the cutaneous skeleton,
from members of the class of Sea-urchins.

From the Star-fish we turn to the fifth and most highly developed tribe
of the invertebrate animals, namely, the phylum of Articulata, or those
with _jointed feet_ (Arthropoda). As has already been remarked, this
tribe corresponds to Linnæus’ class of Insects. It contains four
classes: (1) the genuine six-legged Insects, or Flies; (2) the
eight-legged Spiders; (3) the Centipedes, with numerous pairs of legs;
and (4) the Crabs, or Crustacea, whose legs vary in number. The last
class breathe water through gills, and may therefore be contrasted as
the main-class of gill-breathing Arthropoda, or Gilled Insects
(Carides), with the three first classes. The latter breathe air by means
of peculiar wind-pipes, or tracheæ, and may therefore appropriately be
united to form the main-class of the trachea-breathing Arthropoda, or
Tracheate Insects (Tracheata).

In all animals with articulated feet, as the name indicates, the legs
are distinctly articulated, and by this, as well as by the strong
differentiation of the separate parts of the body, or metamera, they are
sharply distinguished from Ringed worms, with which Bär and Cuvier
classed them. They are, however, in every respect so like the Ringed
worms that they can scarcely be considered altogether distinct from
them. They, like the Ringed worms, possess a very characteristic form of
the central nervous system, the so-called ventral marrow, which
commences in a gullet-ring encircling the mouth. From other facts also,
it is evident that the Arthropoda developed at a late period out of
articulated worms. Probably either the Wheel Animalcules or the Ringed
worms are their nearest blood relations in the Worm tribe. (Gen. Morph.
ii. Plate V. pp. 85-102.)

Now, although the derivation of the Arthropoda from ringed Worms may be
considered as certain, still it cannot with equal assurance be
maintained that the whole tribe of the former has arisen out of one
branch of the latter. For several reasons seem to support the
supposition that the Gilled Arthropods have developed out of a branch of
articulated worms, different from that which gave rise to the Tracheate
Arthropods. But on the whole it remains more probable that both
main-classes have arisen out of one and the same group of Worms. In this
case the Tracheate Insects—Spiders, Flies, and Centipedes—must have
branched off at a later period from the gill-breathing Insects, or
Crustacea.

The pedigree of the Arthropoda can on the whole be clearly made out from
the palæontology, comparative anatomy, and ontogeny of its four
classes, although here, as everywhere else, many details remain very
obscure. Not until the history of the individual development of all the
different groups has become more accurately known than it is at present,
can this obscurity be removed. The history of the class of Gilled
Insects, or Crabs (Carides), is at present that best known to us; they
are also called encrusted animals (Crustacea), on account of the hard
crust or covering of their body. The ontogeny of these animals is
extremely interesting and, like that of Vertebrate animals, distinctly
reveals the essential outlines of the history of their tribe, that is,
their phylogeny. Fritz Müller, in his work, “Für Darwin,”(16) which has
already been referred to, has explained this remarkable series of facts
in a very able manner.

[Illustration: _Nauplius. Youth-form of six Crab-fish._ _Pl. X._

  A. Limnetis.
  B. Cyclops.
  C. Lernacocera.
  D. Lepas.
  E. Sacculina.
  F. Peneus.]

[Illustration: _Adult form of the same six Crab-fish._ _Pl. XI._

  A. Limnetis.
  B. Cyclops.
  C. Lernacocera.
  D. Lepas.
  E. Sacculina.
  F. Peneus.]

The common primary form of all Crabs, which in most cases is even now
the first to develop out of the egg, is originally one and the same, the
so-called _Nauplius_. This remarkable primæval crab represents a very
simple form of articulated animal, the body of which in general has the
form of a roundish, oval, or pear-shaped disc, and has on its ventral
side only three pairs of legs. The first of these is uncloven, the two
subsequent pairs are forked. In front, above the mouth, lies a simple,
single eye. Although the different orders of the Crustacean class differ
very widely from one another in the structure of their body and its
appendages, yet the early Nauplius form always remains essentially the
same. In order to be convinced of this, let the reader look attentively
at Plates X. and XI., a more detailed explanation of which is given in
the Appendix. On Plate XI. we see the fully developed representatives of
six different orders of Crabs, a Leaf-footed Crab (Limnetis, Fig. _A
c_); a Stalked Crab (Lepas, Fig. _D c_); a Root Crab (Sacculina, Fig.
_E c_); a Boatman Crab (Cyclops, Fig. _B c_); a Fish Louse (Lernæocera,
Fig. _C c_); and, lastly, a highly developed Shrimp (Peneus, Fig. _F
c_). These six crabs vary very much, as we see, in the entire form of
body, in the number and formation of the legs, etc. When, however, we
look at the earliest stages, or “nauplius,” of these six different
classes, after they have crept out of the egg—those marked with
corresponding letters on Plate X. (Fig. _A n-F n_)—we shall be surprised
to find how much they agree. The different forms of Nauplius of these
six orders differ no more from one another than would six different
“good species” of one genus. Consequently, we may with assurance infer a
common derivation of all those orders from a common Primæval Crab, which
was essentially like the Nauplius of the present day.

SYSTEMATIC SURVEY

_Of the 7 Legions and 20 Orders of Crabs, or Crustacea._

  -------------------------+--------------------------+----------------------+--------------
     _Legions of the_      |     _Orders of the_      |    _Systema  Name_   |  _Name of a_
      _Crustaceæ._         |      _Crustaceæ._        |    _of the Orders._  | _Genus as an_
                           |                          |                      |  _example._
  -------------------------+--------------------------+----------------------+--------------
  I. ENTOMOSTRACA, _Lower Crustacea_, or Segmented Crabs (not passing through the
                             actual Zoëa form in youth).
  ------------------------------------------------------------------------------------------

  +I. Branchiopoda+         { 1. Primæval Crabs         1. Archicarida         Nauplius
  Gill-footed Crabs         { 2. Leaf-foot Crabs        2. Phyllopoda          Limnetis
                            { 3. Trilobites             3. Trilobita           Paradoxides
                            { 4. Water Fleas            4. Cladocera           Daphnia
                            { 5. Bivalve Crabs          5. Ostracoda           Cypris

  +II. Pectostraca+         { 6. Barnacle Crabs         6. Cirripedia          Lepas
    Fixed Crabs             { 7. Root Crabs             7. Rhizocephala        Sacculina

  +III. Copepoda+           { 8. Boatmen Crabs          8. Eucopepoda          Cyclops
  Oar-footed Crabs          { 9. Fish Lice              9. Siphonostoma        Lernæocera

  +IV. Pantopoda+           {10. No-body Crabs         10. Pycnogonida         Nymphon
   No-body Crabs            {

  +V. Pœcilopoda+           {11. Spear-tails           11.  Xiphosura          Limulus
   Shield Crabs             {12. Giant Crabs           12.  Gigantostraca      Eurypterus

  ------------------------------------------------------------------------------------------
  II. MALACOSTRACA, _Higher Crustacea_, or Mailed Crabs (passing through the Zoëa form
                                     in youth).
  ------------------------------------------------------------------------------------------

  +VI. Podophthalma+        {13. Zoëa Crabs            13. Zoëpoda            Zoëa
  Stalk-eyed Mailed         {14. Split-legged Crabs    14. Schizopoda         Mysis
  Crabs                     {15. Mouth-footed Crabs    15. Stomatopoda        Squilla
                            {16. Ten-footed Crabs      16. Decapoda           Peneus

  +VII. Edriophthalma+      {17. Cuma Crabs            17. Cumacea            Cuma
   Mailed Crabs with        {18. Flea Crabs            18. Amphipoda          Gammarus
    sessile eyes            {19. Wizard Crabs          19. Læmodipoda         Caprella
                            {20. Louse Crabs           20. Isopoda            Oniscus

        ----------------------------------------------------------------

  Brachyura                                            Isopoda
      |                                                   |
      |                                    Læmodipoda     |
      |                                        |          |
  Anomura                                      |          |
      |                                        |          |
      |                                     Amphipoda     |
      |                                        |          |
      |                                        \-----v----/
  Macrura                                           |
  Decapoda  Stomatopoda                          Cumacea
      |         |                            +Edriophthalma+
      |         |                                   |
      \----v----/                                   |
           |                                        |
           |                                        |
           \---------------------------v------------/
                                    Schizopoda
                                  +Podophthalma+
                                    Zoëpoda
                                +Malacostraca+
  Gigantostraca                        |                 Rhizocephala
         |                             |  Siphonostoma        |
         |         Xiphosuræ         Zoëa      |          Cirripediæ
         |             |               |       |         +Pectostraca+
         |             |               |       |              |
         \------v------/               |       |              |
                |                      |       |              |
        +Pœcilopoda+                   |       |              |
                |            Nebaliæ   |       |              |
                |               |      |  Eucopepoda          |
            Belinurœ            |      |  +Copepoda+          |     Pycnogonida
                |               |      |       |              |     +Pantopoda+
            Trilobita           |      |       |              |          |
                |           Phyllopoda |       |  Ostracoda   |          |
  Cladoceræ     |               |      |       |      |       |          |
      |         |               |      |       |      |       |          |
      \-----------------v-------/      |       |      \---v---/          |
                        |              |       |          |              |
                 +Branchiopoda+        |       |          |              |
                        |              |       |          |              |
                        |              |       |          |              |
                        \----------------------v-------------------------/
                                            Nauplius
                                          +Archicaridæ+

                                        (Articulated Worms)

The pedigree on p. 177 will show how we may at present approximately
conceive the derivation of the twenty orders of Crustacea enumerated on
p. 176, from the common primary form of the Nauplius. Out of the
Nauplius form—which originally existed as an independent genus—the five
legions of lower Crabs developed as diverging branches in different
directions, which in the systematic survey of the class are united as
Segmented Crabs (Entomostraca). The higher division of Mailed Crabs
(Malacostraca) have likewise originated out of the common Nauplius form.
The Nebalia is still a direct form of transition from the Phyllopods to
the Schizopods, that is, to the primary form of the stalk-eyed and
sessile-eyed Mailed Crabs. The Nauplius at this stage gives rise to
another larva form, the so-called Zoëa, which is of great importance.
The order of Schizopoda, those with cloven feet (Mysis, etc.), probably
originated from this curious Zoëa; they are at present still directly
allied, through the Nebalia to the Phyllopoda, those with foliaceous
feet. But of all living crabs the Phyllopods are the most closely allied
to the original primary form of the Nauplius. Out of the Schizopoda the
stalk-eyed and sessile-eyed Mailed Crabs, or Malacostraca, developed as
two diverging branches in different directions: the former through
shrimps (Peneus, etc.), the latter through the Cumacea (Cuma, etc.),
which are still living and closely allied to the Schizopoda. Among those
with stalked eyes is the river crab (cray-fish), the lobster, and the
others with long tails, or the Macrura, out of which, in the chalk
period, the short-tailed crabs, or Brachyura, developed by the
degeneration of the tail. Those with sessile eyes divide into the two
branches of Flea-crabs (Amphipoda) and Louse-crabs (Isopoda); among the
latter are our common Rock-slaters and Wood-lice.

The second main-class of Articulated animals, that of the Tracheata, or
air-breathing Tracheate Insects[4] (Spiders, Centipedes, and Flies) did
not develop until the beginning of the palæolithic era, after the close
of the archilithic period, because all these animals (in contrast with
the aquatic crabs) are originally inhabitants of land. It is evident
that the Tracheata can have developed only after the lapse of the
Silurian period when terrestrial life first began. But as fossil remains
of spiders and insects have been found, even in the carboniferous beds,
we can pretty accurately determine the time of their origin. The
development of the first Tracheate Insects out of gill-bearing
Zoëa-crabs, must have taken place between the end of the Silurian and
the beginning of the coal period, that is, in the Devonian period.

Gegenbaur, in his excellent “Outlines of Comparative Anatomy,”(21) has
lately endeavoured to explain the origin of the Tracheata by an
ingenious hypothesis. The system of tracheæ, or air pipes, and the
modifications of organization dependent upon it, distinguish Flies,
Centipedes, and Spiders so much from other animals, that the conception
of its first origin presents no inconsiderable difficulties to
phylogeny. According to Gegenbaur, of all living Tracheate Insects, the
Primæval Flies, or Archiptera, are most closely allied to the common
primary form of the Tracheata. These insects—among which we may
especially mention the delicate Day flies (Ephemera), and the agile
dragon-flies (Libellula)—in their earliest youth, as larvæ, frequently
possess _external tracheate gills_ which lie in two rows on the back of
the body, and are shaped like a leaf or paint-brush. Similar leaf or
paint-brush shaped organs are met with as real water-breathing organs or
gills, in many crabs and ringed worms, and, moreover, in the latter as
real dorsal appendages or limbs. The “tracheate gills,” found in the
larvæ of many primæval winged insects, must in all probability be
explained as “_dorsal limbs_,” and as having developed out of the
corresponding appendages of the Annelida, or possibly as having really
arisen out of similar parts in Crustacea long since extinct. The present
tracheal respiration of the Tracheata developed at a later period out of
respiration through the “tracheate gills.” The tracheate gills
themselves, however, have in some cases disappeared, and in others
become transformed into the _wings_ of the Flies. They have disappeared
entirely in the classes of Spiders and Centipedes, and these groups must
accordingly be conceived of as degenerated or peculiarly developed
lateral branches of the Fly class, which at an early period branched off
from the common primary form of Flies; Spiders probably did so at an
earlier period than Centipedes. Whether that common primary form of all
Tracheata, which in my General Morphology I have named Protracheata, did
develop directly out of genuine Ringed worms, or at first out of
Crustacea of the Zoëa form (Zoëpoda, p. 177) will probably be settled at
some future time by a more accurate knowledge and comparison of the
ontogeny of the Tracheata, Crustacea, and Annelida. However, the root of
the Tracheata, as well as that of the Crustacea, must in any case be
looked for in the group of Ringed worms.

The genuine Spiders (Arachnida) are distinguished from Flies by the
absence of wings, and by four pairs of legs; but, as is distinctly seen
in the Scorpion-spiders and Tarantulæ, they, like Flies, possess in
reality only three pairs of genuine legs. The apparent “fourth pair of
legs” in spiders (the foremost) are in reality a pair of feelers. Among
the still existing Spiders, there is a small group which is probably
very closely allied to the common primary form of the whole class; this
is the order of Scorpion-spiders, or Solifugæ, (Solpuga, Galeodes), of
which several large species live in Africa and Asia, and are dreaded on
account of their poisonous bite. Their body consists—as we suppose to
have been the case in the common ancestor of the Tracheata—of a head
possessing several pairs of feelers like legs, of a thorax, to the three
rings of which are attached three pairs of legs, and of a hinder body,
or abdomen, consisting of many distinct rings. In the articulation of
their body, the Solifugæ are therefore in reality more closely related
to flies than to other spiders. Out of the Devonian Primæval Spiders,
which were nearly related to the Solifugæ of the present day, the Long
Spiders, the Tailor Spiders, and the Round Spiders probably developed as
three diverging branches.

The _Long Spiders_ (Arthrogastres), in which the earlier articulation of
body has been better preserved than in Round Spiders, appear to be the
older and more original forms. The most important members of this
sub-class are the scorpions, which are connected with the Solifugæ
through the Tarantella (or Phrynidæ). The small book scorpions, which
inhabit our libraries and herbariums, appear as a degenerate lateral
branch from the true scorpions. Midway between the Scorpions and Round
Spiders are the long-legged Tailor-spiders (Opiliones) which have
possibly arisen out of a special branch of the Solifugæ. The
Pycnogonida, or No-body Crabs, and the Arctisca, or Bear Worms—still
generally included among Long Spiders—must be completely excluded from
the class of Spiders; the former belong to the Crustacea, the latter to
Ringed worms.

SYSTEMATIC SURVEY

_Of the 3 Classes and 17 Orders of the Tracheata._

  ------------------+---------------------+----------------------+----------------------
  _Classes of the_  | _Sub-Classes of the_|   _Order of the_     |   _Two Names of_
    _Tracheata._    |     _Tracheata._    |     _Tracheata._     |_Genera as examples._
  ------------------+---------------------+----------------------+----------------------
                    {                     {  1. Scorpion spiders  { Solpuga
                    {                     {    _Solifugæ_         { Galeodes
                    {                     {
                    {                     {  2. Tarantella        { Phrynus
                    {                     {    _Phrynida_         { Thelyphonus
                    {         I.          {
           I.       {     Long spiders    {  3. Scorpions         { Scorpio
        =Spiders=   {    _Arthrogastres_  {    _Scorpioda_        { Buthus
                    {                     {
       +Arachnida+  {                     {  4. Book scorpions    { Obisium
                    {                     {    _Pseudoscorpioda_  { Chelifer
                    {                     {
                    {                     {  5. Tailor spiders    { Phalangium
                    {                     {    _Opilionida_       { Opilio
                    {
                    {
                    {         II.         {  6. Spinning spiders  { Epeira
                    {     Round spiders   {    _Araneæ_           { Mygale
                    {                     {                       {
                    {    _Sphærogastres_  {  7. Mites             { Sarcoptes
                    {                     {    _Acarida_          { Demodex

          II.       {          III.
      =Centipedes=  {      Simple-footed  {  8. Simple-footed     { Scolopendra
                    {       _Chilopoda_   {    _Chilopoda_        { Geophilus
                    {                     {
     +Scolopendria+ {          IV.        {  9. Double-footed     { Julus
           or       {      Double-footed  {    _Diplopoda_        { Polydesmus
       +Myriapoda+  {       _Diplopoda_   {

                    {                     { 10. Primitive flies   { Ephemera
                    {                     {    _Archiptera_       { Libellula
                    {                     {
                    {                     { 11. Gauze-wings       { Hemerobius
                    {                     {    _Neuroptera_       { Phryganea
                    {           V.        {
                    {        Chewing      { 12. Straight-wings    { Locusta
                    {     _Masticantia_   {    _Orthoptera_       { Forficula
                    {                     {
          III.      {                     { 13. Beetles           { Cicindela
         =Flies=    {                     {    _Coleoptera_       { Melolontha
                    {                     {
       +Hexapoda+   {                     { 14. Bee-wings         { Apis
                    {                     {    _Hymenoptera_      { Formica
                    {
                    {
                    {                     { 15. Bugs              { Aphis
                    {          VI.        {    _Hemiptera_        { Cimex
                    {        Sucking      {
                    {      _Sugentia_     { 16. Two-wings         { Culex
                    {                     {    _Diptera_          { Musca
                    {                     {
                    {                     { 17. Butterflies       { Bombyx

        ----------------------------------------------------------------

                                Butterflies
                               _Lepidoptera_
                     Bees            |           Two-wings
                 _Hymenoptera_       |           _Diptera_
                       |             |               |
                       |             |               |
      Beetles          |             |               |             Bugs
  _Coleoptera_         \-------v-----/               |          _Hemiptera_
        |                      |                     |              |
        |                 Gauze wings                |              |
  Straight-wings          _Neuroptera_               \-------v------/
  _Orthoptera_                 |                             |
        |                      |                             |
        |                      |                             |
        \----------------------v-----------------------------/

                                         Primæval Flies
                                          _Archiptera_
                                  Scorpions    |             Double-footed
                                 _Scorpioda_   |              _Diplopoda_
             Tailor Spiders            |       |                   |
              _Opiliones_              |       |                   |
                   |  Book Scorpions   |       |                   |
       Mites       | _Pseudoscorpioda_ |       |                   |
      _Acarida_    |       |           |       |                   |
          |        |       \-----v-----/       |                   |
          |        |             |             |                   |
          |        |         Tarantella        |                   |
   Weaving Spiders |         _Phrynida_        |              Simple-footed
      _Araneæ_     |             |             |               _Chilopoda_
          |        |             |             |               =Centipedes=
          |        |             |             |               +Myriapoda+
          \-------------v--------/             |                   |
                        |                      |                   |
                 Scorpion Spiders              |                   |
                   _Solifugæ_                  |                   |
                    =Spiders.=                 |                   |
                   +Arachnida+                 |                   |
                        |                      |                   |
                        |                    =Flies=               |
                        |               +Insecta Hexapoda+         |
                        |                      |                   |
                        |                      |                   |
                        \--------------------v---------------------/
                               Primary Air-breathing Arthropods
                                        _Protracheata_
                                      Articulated Worms
                                        _Coelminthes_

Fossil remains of Long Spiders are found in the Coal. The second
sub-class of the Arachnida, the _Round Spiders_ (Sphærogastres), first
appear in the fossil state in the Jura, that is, at a very much later
period. They have developed out of a branch of the Solifuga, by the
rings of the body becoming more and more united with one another. In the
true _Spinning Spiders_ (Araneæ), which we admire on account of their
delicate skill in weaving, the union of the joints of the trunk, or
metamera, goes so far, that the trunk now consists of only two pieces,
of a head-breast (cephalo-thorax) with jaws, feelers, and four pairs of
legs, and of a hinder body without appendages, where the spinning warts
are placed. In _Mites_ (Acarida), which have probably arisen by
degeneration (especially by parasitism) out of a lateral branch of
Spinning Spiders, even these two trunk pieces have become united and now
form an unsegmented mass.

The class of _Scolopendria_, _Myriapoda_, or Centipedes, the smallest
and poorest in forms of the four classes of Arthropoda, is characterized
by a very elongated body, like that of a segmented Ringed worm, and
often possesses more than a hundred pairs of legs. But these animals
also originally developed out of a six-legged form of Tracheata, as is
distinctly proved by the individual development of the millipede in the
egg. Their embryos have at first only three pairs of legs, like genuine
insects, and only at a later period do the posterior pairs of legs bud,
one by one, from the growing rings of the hinder body. Of the two orders
of Centipedes (which in our country live under barks of trees, in moss,
etc.) the round, _double-footed_ ones (Diplopoda) probably did not
develop until a later period out of the older flat, _single-footed_ ones
(Chilopoda), by successive pairs of rings of the body uniting together.
Fossil remains of the Chilopoda are first met with in the Jura period.

The third and last class of the Arthropoda breathing through tracheæ, is
that of the _Flies_, or _Insects_, in the narrow sense of the word
(Insecta, or Hexapoda), the largest of all classes of animals, and next
to that of Mammalia, also the most important. Although Flies develop a
greater variety of genera and species than all other animals taken
together, yet these are all in reality only superficial variations of a
single type, which is entirely and constantly preserved in its essential
characteristics. In all Flies the three divisions of the trunk—head,
breast (thorax), and hinder body are quite distinct. The _hinder body_,
or _abdomen_, as in the case of spiders, has no articulated appendages.
The central division, the _breast_ or _thorax_, has on its ventral side
three pairs of legs, on its back _two pairs of wings_. It is true that,
in very many Flies, one or both pairs of wings have become reduced in
size or have even entirely disappeared; but the comparative anatomy of
Flies distinctly shows that this deficiency has arisen only gradually by
the degeneration of the wings, and that all the Flies existing at
present are derived from a common, primary Fly, which possessed three
pairs of legs and two pairs of wings. (Compare p. 256.) These wings,
which so strikingly distinguish Flies from all other Arthropoda,
probably arose, as has been already shown, out of the tracheate gills
which may still be observed in the larvæ of the ephemeral flies
(Ephemera) which live in water.

The head of Flies universally possesses, besides the eyes, a pair of
articulated feelers, or antennæ, and also three jaws upon each side of
the mouth. These _three pairs of jaws_, although they have arisen in all
Flies from the same original basis, by different kinds of adaptation,
have become changed to very varied and remarkable forms in the various
orders, and are therefore employed for distinguishing and characterizing
the main divisions of the class. In the first place, we may distinguish
two main divisions, namely, Flies with _chewing_ mandibles (Masticantia)
and Flies with _sucking_ mouths (Sugentia). On a closer examination each
of these two divisions may again be divided into two sub-groups. Among
chewing Flies, or Masticantia, we may distinguish the biting and the
licking ones. _Biting flies_ (Mordentia) comprise the most ancient and
primæval winged Flies, the gauzy-winged (Neuroptera), straight-winged
(Orthoptera), and beetles (Coleoptera). _Licking flies_ (Lambentia) are
represented by the one order of skin-winged (Hymenoptera) Flies. We
distinguish two groups of _Sucking Flies_, or Sugentia, namely, those
which prick and those which sip. There are two orders of pricking Flies
(Pungentia), those with half wings (Hemiptera) and gnats and blow-flies
(Diptera); butterflies are the only _sipping_ Flies (Sorbentia),
Lepidoptera.

Biting Flies, and indeed the order of _Primæval Flies_ (Archiptera, or
Pseudoneuroptera) are nearest akin to the still living Flies, and
include the most ancient of all Flies, the primary forms of the whole
class (hence also those of all Tracheata). Among them are, first of all,
the Ephemeral Flies (Ephemera) whose larvæ which live in water, in all
probability still show us in their tracheæ-gills the organs out of which
the wings of Flies were originally developed. This order further
contains the well known dragon-flies, or Libellula, the wine-glass sugar
mites (Lepisma), the hopping Flies with bladder-like feet (Physopoda),
and the dreaded Termites, fossil remains of which are found even in
coal. The order of Gauze-winged Flies (Neuroptera), probably developed
directly out of the primæval Flies, which differ from them only by their
perfect series of transformations. Among them are the gauze-flies
(Planipennia), caddis-flies (Phryganida), and fan-flies (Strepsiptera).
Fossil Flies, which form the transition from the primæval Flies
(Libellula) to the gauze-winged (Sialidæ), are found even in coal
(Dictyophylebia).

The order of _Straight-winged Flies_ (Orthoptera) developed at an early
period out of another branch of the primæval Flies by differentiation of
the two pairs of wings. This division is composed of one group with a
great variety of forms—cockroaches, grasshoppers, crickets, etc.
(Ulonata)—and of a smaller group consisting only of the well-known
earwigs (Labidura), which are characterised by nippers at the hinder end
of their bodies. Fossil remains of cockroaches, as well as of crickets
and grasshoppers, have been found in coal.

Fossil remains of the fourth order of Biting Flies, _beetles_
(Coleoptera) likewise occur in coal. This extremely comprehensive
order—the favourite one of amateurs and collectors—shows more clearly
than any other what infinite variety of forms can be developed
externally by adaptation to different conditions of life, without the
internal structure and the original form of the body being in any way
essentially changed. Beetles have probably developed out of a branch of
the straight-winged Flies, from which they differ only in their
transformations (larva, pupa, etc.).

The one order of _Licking Flies_, namely, the interesting group of the
_Bees_, or _Skin-winged Flies_ (Hymenoptera), is closely allied to the
four orders of biting Flies. Among them are those Flies which have risen
to such an astonishing degree of mental development, of intellectual
perfection, and strength of character, by their extensive division of
labour, formation of communities and states, and surpass in this not
merely most invertebrate animals, but even most animals in general. This
may be said especially of all ants and bees, also of wasps, leaf-wasps,
wood-wasps, gall-wasps, etc. They are first met with in a fossil state
in the oolites, but they do not appear in greater numbers until the
tertiary period. Probably these insects developed either out of a branch
of the primæval Flies or the gauze-winged Flies.

Of the two orders of _Pricking Flies_ (Hemiptera and Diptera), that
containing the _Half-winged Flies_ (Hemiptera), also called Beaked Flies
(Rhynchota), is the older of the two. It includes three sub-orders,
viz., the leaf-lice (Homoptera), the bugs (Heteroptera), and lice
(Pediculina). Fossil remains of the first two classes are found in the
oolites; but an ancient Fly (Eugereon) is found in the Permian system,
and seems to indicate the derivation of the Hemiptera from the
Neuroptera. Probably the most ancient of the three sub-orders of the
Hemiptera are the Homoptera, among which, besides the actual leaf-lice,
are the shield-lice, leaf-fleas, and leaf-crickets, or Cicadæ. Lice have
probably developed out of two different branches of Homoptera, by
continued degeneration (especially by the loss of wings); bugs, on the
other hand, by the perfecting and differentiation of the two pairs of
wings.

The second order of _pricking flies_, namely, the _Two-winged Flies_
(Diptera), are also found in a fossil state in the oolites, together
with Half-winged Flies; but they probably developed out of the Hemiptera
by the degeneration of the hind wings. In Diptera the fore wings alone
have remained perfect. The principal portion of this order consists of
the elongated gnats (Nemocera) and of the compact blow-flies and
house-flies (Brachycera), the former of which are probably the older of
the two. However, remains of both are found in the oolitic period. The
two small groups of lice-flies (Pupipara) forming chrysales, and the
hopping-fleas (Aphaniptera), probably developed out of the Diptera by
degeneration resulting from parasitism.

The eighth and last order of Flies, and at the same time the only one
with mouth-parts adapted to sipping liquids, consists of _moths_ and
_butterflies_ (Lepidoptera). This order appears, in several
morphological respects, to be the most perfect class of Flies, and
accordingly was the last to develop. For we only know of fossil remains
of this order from the tertiary period, whereas the three preceding
orders extend back to the oolites, and the four biting orders even to
the coal period. The close relationship between some moths (Tineæ) and
(Noctuæ), and some caddis-flies (Phryganida) renders it probable that
butterflies have developed from this group, that is, out of the order of
Gauze-winged Flies, or Neuroptera.

The whole history of Flies, and, moreover, the history of the whole
tribe of Arthropoda, essentially confirms the great laws of
differentiation and perfecting which, according to Darwin’s theory of
selection, must be considered as the necessary results of Natural
Selection. The whole tribe, so rich in forms, begins in the Archilithic
period with the class of _Crabs_ breathing by gills, and with the lowest
_Primæval Crabs_, or Archicaridæ. The form of these Primæval Crabs,
which were developed out of segmented worms, is still approximately
preserved by the remarkable _Nauplius_, in the common larval stage of so
many Crabs. Out of the Nauplius, at a later period, the curious Zoëa was
developed, which is the common larval form of all the higher or mailed
crabs (Malacostraca), and, at the same time, possibly of that Arthopod
which at first breathed through tracheæ, and became the common ancestor
of all _Tracheata_. This Devonian ancestor, which must have originated
between the end of the Silurian and the beginning of the Coal period,
was probably most closely related to the still living Primæval Flies, or
_Archiptera_. Out of these there developed, as the main tribe of the
Tracheata, the class of Flies, from the lowest stage of which the
_spiders_ and _centipedes_ separated as two diverging branches.
Throughout a long period there existed only the four biting orders of
Flies—the Primæval flies, Gauze-wings, Straight-wings, and the Beetles,
the first of which is probably the common primary form of the three
others. It was only at a much later period that the Licking, Pricking,
and Sipping flies developed out of the Biting ones, which retained the
original form of the three pairs of jaws most distinctly. The following
table will show once more how these orders succeeded one another in the
history of the earth.

CLASSIFICATION OF FLIES.

                    {                 { 1. Primæval winged   { M.I.  }
                    {                 {    _Archiptera_      { A.A.  }
                    {                 {                              }
                    {      I.         { 2. Gauze-winged      { M.C.  }
        A.          {  Biting Flies   {    _Neuroptera_      { A.A.  }
                    {                 {                              }
      =Flies=       {  _Mordentia_    { 3. Straight-winged   { M.I.  }
  =with Chewing=    {                 {    _Orthoptera_      { A.D.  }
     =Mouths=       {                 {                              }
                    {                 { 4. Beetles           { M.C.  }
  +Masticantia+     {                 {    _Coleoptera_      { A.D.  }
                    {
                    {
                    {      II.        { 5. Skin-winged       { M.C.  }
                    {  Licking Flies  {    _Hymenoptera_     { A.A.  }
                    {   _Lambentia_   {                              }
                                                                     }
                                                                     }
                    {                 { 6. Half-winged       { M.I.  }
                    {      III.       {    _Hemiptera_       { A.A.  }
        B.          {  Stinging Flies {                              }
      =Flies=       {                 { 7. Tway-flies        { M.C.  }
  =with Sucking=    {   _Pungentia_   {    _Diptera_         { A.D.  }
     =Mouths=       {
                    {
   +Sugentia+       {      IV.        { 8. Butterflies       { M.C.  }
                    {  Sipping Flies  {    _Lepidoptera_     { A.A.  }
                    {                 {                      {       }
                    {   _Sorbentia_   {                      {       }

   _Note._—The difference in the metamorphosis or transformation and in
   the development of the wings of the eight individual orders of Flies
   is also specified by the following letters: M.I. = Imperfect
   Metamorphosis. M.C. = Perfect Metamorphosis. (Compare Gen. Morph. ii.
   p. 99.) A.A. = Equal wings (fore and hinder wings are the same, or
   differ but little). A.D. = Unequal wings (fore and hinder wings very
   different in structure and texture, occasioned by strong
   differentiation).

CHAPTER XX.

PEDIGREE AND HISTORY OF THE ANIMAL KINGDOM.

III. VERTEBRATE ANIMALS.

  The Records of the Creation of Vertebrate Animals (Comparative
  Anatomy, Embryology, and Palæontology).—The Natural System of
  Vertebrate Animals.—The Four Classes of Vertebrate Animals,
  according to Linnæus and Lamarck.—Their increase to Nine
  Classes.—Main Class of the Tube-hearted, or Skull-less Animals
  (the Lancelet).—Blood Relationship between the Skull-less Fish
  and the Tunicates.—Agreement in the Embryological Development
  of Amphioxus and Ascidiæ.—Origin of the Vertebrate Tribe out of
  the Worm Tribe.—Main Class of Single-nostriled, or
  Round-mouthed Animals (Hag and Lampreys).—Main Class of
  Anamnionate Animals, devoid of Amnion.—Fishes (Primæval Fish,
  Cartilaginous Fish, Osseous Fish).—Mud-fish, or Dipneusta.—Sea
  Dragons, or Halisauria.—Frogs and Salamanders, or Amphibia
  (Mailed Amphibia, Naked Amphibia).—Main Class of Amnionate
  Animals, or Amniota.—Reptiles (Primary Reptiles, Lizards,
  Serpents, Crocodiles, Tortoises, Flying-Reptiles, Dragons,
  Beaked Reptiles).—Birds (Feather-tailed, Fan-tailed,
  Bush-tailed).

Not one of the natural groups of organisms—which, we have designated as
tribes, or phyla, on account of the blood-relationship of all the
species included in them—is of such great and exceeding importance as
the tribe of Vertebrate Animals. For, according to the unanimous opinion
of all zoologists, man also is a member of the tribe; and his whole
organization and development cannot possibly be distinguished from that
of other Vertebrate animals. But as from the individual history of
human development, we have already recognized the undeniable fact that,
in developing out of the egg, man at first does not differ from other
Vertebrate animals, and especially from Mammals, we must necessarily
come to the conclusion, in regard to the palæontological history of his
development, that man has, historically, actually developed out of the
lower Vertebrata, and that he is directly derived from lower mammals.
This circumstance, together with the many high interests which, in other
respects, entitle the Vertebrata to more consideration than other
organisms, justifies us in examining the pedigree of the Vertebrata and
its expression in the natural system, with special care.

Fortunately, the records of creation, which must in all cases be our
guide in establishing pedigrees, are especially complete in this
important animal tribe, from which our own race has arisen. Even at the
beginning of our century Cuvier’s comparative anatomy and palæontology,
and Bär’s ontogeny of the Vertebrate animals, had brought us to a high
level of accurate knowledge on this matter. Since then it is especially
due to Johannes Müller’s and Rathke’s investigations in comparative
anatomy, and most recently to those of Gegenbaur and Huxley, that our
knowledge of the natural relationships among the different groups of
Vertebrata has become enlarged. It is especially Gegenbaur’s classical
works, penetrated as they are throughout with the fundamental principles
of the Theory of Descent, which have demonstrated that the material of
comparative anatomy receives its true importance and value only by the
application of the Theory of Descent, and this in the case of all
animals, but especially in that in the Vertebrate tribe. Here, as
everywhere else, analogies must be traced to Adaptation, homologies to
Transmission by Inheritance. When we see that the limbs of the most
different Vertebrata, in spite of their exceedingly different external
forms, nevertheless possess essentially the same internal structure;
when we see that in the arm of a man and ape, in the wing of a man or a
bird, in the breast fins of whales and sea-dragons, in the fore-legs of
hoofed animals and frogs, the same bones always lie in the same
characteristic position, articulation and connection—we can only explain
this wonderful agreement and homology by the supposition of a common
transmission by inheritance from a single primary form. On the other
hand, the striking differences of these homologous bodily parts proceed
from adaptation to different conditions of existence. (Compare Plate
IV.)

Ontogeny, or the individual history of development, like comparative
anatomy, is of especial importance to the pedigree of the Vertebrata.
The first stages of development arising out of the egg are essentially
identical in all Vertebrate animals, and retain their agreement the
longer, the nearer the respective Vertebrate animal forms, when fully
developed, stand to one another in the natural system, that is, in the
pedigree. How far this agreement of germ forms, or embryos, extends,
even in the most highly developed Vertebrate animals, I have already had
occasion to explain (vol i. pp. 306-309). The complete agreement in form
and structure, for example, in the embryos of a man and a dog, of a bird
and a tortoise, existing in the stages of development represented on
Plates II. and III., is a fact of incalculable importance, and furnishes
us with the most important data for the construction of their pedigree.

Finally, the palæontological records of creation are also of especial
value in the case of these same Vertebrate animals; for their fossil
remains belong for the most part to the bony skeleton, a system of
organs which is of the utmost importance for understanding their general
organization. It is true that here, as in all other cases, the fossil
records are exceedingly imperfect and incomplete, but more important
remains of extinct Vertebrate animals have been preserved in a fossil
state, than of most other groups of animals; and single fragments
frequently furnish the most important hints as to the relationship and
the historical succession of the groups.

The name of _Vertebrate Animals_ (Vertebrata), as I have already said,
originated with the great Lamarck, who towards the end of the last
century comprised under this name, Linnæus’ four higher classes of
animals, viz. Mammals, Birds, Amphibious animals, and Fishes, Linnæus’
two lower classes, Insects and Worms, Lamarck contrasted to the
Vertebrata as _Invertebrata_, later also called _Evertebrata_.

The division of the Vertebrata into the four classes above named was
retained also by Cuvier and his followers, and in consequence by many
zoologists down to the present day. But in 1822 Blanville, the
distinguished anatomist, found out by comparative anatomy—which Bär did
almost at the same time from the ontogeny of Vertebrata—that Linnæus’
class of Amphibious animals was an unnatural union of two very different
classes. These two classes were separated as early as 1820, by Merrin,
as two main groups of Amphibious animals, under the names of Pholidota
and Batrachia. The _Batrachia_, which are at present (in a restricted
sense) called Amphibious animals, comprise Frogs, Salamanders, gilled
Salamanders, Cæcilia, and the extinct Labyrinthodonta. Their entire
organization is closely allied to that of Fishes. The _Pholidota_, or
Reptiles, on the other hand, are much more closely allied to Birds. They
comprise lizards, serpents, crocodiles, and tortoises, and the groups of
the mesolithic Dragons, Flying reptiles, etc.

In conformity with this natural division of Amphibious animals into two
classes, the whole tribe of Vertebrate animals was divided into two main
groups. The first main group, containing Amphibious animals and Fishes,
breathe throughout their lives, or in early life, by means of gills, and
are therefore called _gilled Vertebrata_ (Branchiata, or Anallantoida).
The second main group—Reptiles, Birds, and Mammals—breathe at no period
of their lives through gills, but exclusively through lungs, and hence
may appropriately be called Gill-less, or _Vertebrata with lungs_
(Abranchiata, or Allantoida). However correct this distinction may be,
still we cannot remain satisfied with it if we wish to arrive at a true
natural system of the vertebrate tribe, and at a right understanding of
its pedigree. In this case, as I have shown in my General Morphology, we
are obliged to distinguish three other classes of Vertebrate animals, by
dividing what has hitherto been regarded as the class of fishes into
four distinct classes. (Gen. Morph. vol. ii. Plate VII. pp. 116-160.)

The first and lowest of these classes comprises the _Skull-less_ animals
(Acrania), or animals with _tubular hearts_ (Leptocardia), of which only
one representative now exists, namely, the remarkable little Lancelet
(Amphioxus lanceolatus). Nearly allied to this is the second class, that
of the _Single-nostriled_ animals (Monorrhina), or _Round-mouthed_
animals (Cyclostoma), which includes the Hags (Myxinoida) and Lampreys
(Petromyzonta). The third class contains only the genuine Fish (Pisces):
the Mud-fishes (Dipneusta) are added to these as a fourth class, and
form the transition from Fish to Amphibious animals. This distinction,
which, as will be seen immediately, is very important for the genealogy
of the Vertebrate animals, increases the original number of Vertebrate
classes from four to eight.

In most recent times a ninth class of Vertebrata has been added to these
eight classes. Gegenbaur’s recently published investigations in
comparative anatomy prove that the remarkable class of _Sea-dragons_
(Halisauria), which have hitherto been included among Reptiles, must be
considered quite distinct from these, and as a separate class which
branched off from the Vertebrate stock, even before the Amphibious
animals. To it belong the celebrated large Ichthyosauri and Plesiosauri
of the oolitic and chalk periods, and the older Simosauri of the Trias
period, all of which are more closely allied to Fish than to Amphibious
animals.

These nine classes of Vertebrate animals are, however, by no means of
the same genealogical value. Hence we must divide them, as I have
already shown in the Systematic Survey on p. 133, into four distinct
main-classes or tribes. In the first place, the three highest classes,
Mammals, Birds, and Reptiles, may be comprised as a natural main-class