_Cryptolithus tessellatus_, when the cephalon is 0.45 mm. to 0.65 mm.
long, shows short eye-lines and a small simple eye on each cheek. In
some half-grown specimens, traces of the ocelli can be seen, but the
eye-lines are absent. In the adult, both the eye-lines and the ocelli
are entirely wanting. Reed states that "nervures" are also absent, and
so they are from most specimens, but well preserved casts of the
interior from the Upper Trenton opposite Cincinnati show them, and one
cheek is here figured (fig. 25). As apparent from the figure, the main
trunk is very short and gives rise to two principal branches, the
first of which in its turn sends off lines from the anterior side. It
was a specimen showing these lines which Ruedemann (1916, p. 147)
figured as showing facial sutures. The interest lies in the fact that
while the ocelli and eye-lines were lost in development, the genal
cæca are present in the adult, showing that they are different
structures.

[Illustration: Fig. 25.--_Cryptolithus tessellatus_ Green. Side view
of the cheek of a specimen from the top of the Trenton opposite
Cincinnati, Ohio, to show the branching genal cæca. These are the
"facial sutures" of Ruedemann.]

_Harpides_ is another genus in which genal cæca are strikingly shown,
and in this case they completely cover the huge cheeks, radiating from
two main trunks to the front and sides. I have seen no good specimens,
but it would appear from Angelin's figure (1854, pl. 41, fig. 7) that
the rather large, simple eyes are not situated exactly on the vascular
trunks. In the _Harpides_ from Bohemia, the main trunks extend out
with many branches beyond the simple eyes. It should be stated that
the courses of the genal cæca are not correctly figured by Barrande
(Supplement, 1872, pl. 1, fig. 11), as shown by casts of the original
specimen in the Museum of Comparative Zoology. From Barrande's figure,
one would suppose that the eye-lines and their continuation beyond the
"ocelli" were superimposed upon the genal cæca without having any
definite connection with them, but as a matter of fact the radial
markings really diverge from the main trunks as in _Elyx_ and similar
forms.

_Summary._

As Reed has said, these lines are not mere ornamentation, but rather
represent traces of structures of some functional importance. They
probably can not be explained as traces of nerves and more likely
represent either traces of the gastric cæca or of the circulatory
system. While they are known chiefly in Cambrian and Lower Ordovician
trilobites, there is no evidence that the organs represented were not
present in later forms, even if the shell may not have been affected
by them. While they indicate very fine, thread-like canals, the
present evidence seems to be in favor of assigning to them the
function of lodging the glands which secreted the principal digestive
fluids.

HEART.

_Illænus._

Volborth (1863, pl. 1, fig. 12 = our fig. 26) has described the only
organ in a trilobite which suggests a heart. A Russian specimen of
_Illænus_ with the shell removed shows a somewhat flattened, tubular,
chambered organ extending from under the posterior end of the cephalon
to the anterior end of the pygidium. The posterior nine chambers were
each 1.5 mm. long and 1.5 mm. wide, while the two anterior chambers
were respectively 2.5 mm. and 3 mm. wide. These were all under the
thorax, and at least two more chambers are shown under the cephalon,
but rather obscurely. The species of the _Illænus_ is not stated, but
since no _Illænus_ has more than ten segments in the thorax, and
this tube has at least thirteen chambers, it is evident that its
constrictions are inherent in it, and are not due to the segmentation
of the thorax. Beecher has made a passing allusion to this organ as an
alimentary canal. This was the original opinion of Volborth. Pander,
however, suggested to him that it might be a heart. The alimentary
canal of _Cryptolithus_ does not show any constrictions, while the
heart of _Apus_ (see fig. 27) and other branchiopods does show them.
It should be noted, further, that while this heart enlarges toward the
front, it is everywhere very small as compared with the width of the
axial lobe, and much narrower than sections of _Ceraurus_ and
_Calymene_ would lead one to expect the alimentary canal of _Illænus_
to be. Where the heart is 1.5 mm. to 3 mm. wide, the axial lobe is 11
mm. wide.

[Illustration: Fig. 26. Copy of Volborth's figure of the heart of
_Illænus_.]

[Illustration: Fig. 27. Heart of _Apus_. Copied from Gerstäcker.]

While this may be merely a cast of the alimentary canal it is
sufficiently like a heart to deserve consideration as such an organ.

_Ceraurus and Calymene._

Nothing suggesting a heart has been seen in the sections of _Ceraurus_
and _Calymene_. The mesenteron and its sheath crowd so closely against
the dorsal test in the anterior part of the thorax that there seems
to be no room for the heart, but it must have been located beneath the
sheath and above the alimentary canal. If the latter were filled with
mud, and the animals lay on their backs, as most of them did at death,
the canal would drop down into the axial lobe and the soft heart would
naturally disappear and leave 110 trace of its presence in the
fossils.

_The Median "Ocellus" or "Dorsal Organ."_

Many trilobites, otherwise smooth, bear on the glabella a median
pustule which is usually referred to as a simple eye or median
ocellus, but whose function can not be said to have been certainly
demonstrated. Ruedemann (1916, p. 127), who has recently made a
careful study of this problem, lists about thirty genera, members
of ten families, Agnostidæ, Eodiscidæ Trinucleidæ, Harpedidæ,
Remopleuridæ, Asaphidæ Illænidæ, Goldiidæ, Cheiruridæ, and Phacopidæ,
in which this tubercle is present, and had he wished he might have
cited more, for it is of almost universal occurrence in Ordovician
trilobites.

I have not especially searched the literature for references to this
median tubercle. It is often mentioned by writers in descriptions of
species, but apparently few have tried to explain it. Beyrich (1846,
p. 30) suggested that it indicated the beginning of the alimentary
canal. Barrande mentioned it, but if he gave any explanation, it has
escaped me. McCoy (Syn. Pal. Foss. 1856, p. 146) called it an ocular
(?) tubercle, and that seems to have been the interpretation which
most writers on trilobites have assigned to it, if they suggested any
function at all. Beecher (1895 B, p. 309) concurred in this opinion.

Bernard (1894, p. 422) ascribed to this tubercle, as well as to the
median tubercle on the nuchal segment, an excretory function,
comparing it with the "dorsal organ" in _Apus_.

Reed (1916, p. 174) states that it may be either the representative of
the "dorsal" organ of the branchiopods, or a median unpaired ocellus.

Ruedemann (1916) has made the only real investigation of the subject.
He came to the conclusion that it was a parietal eye, without a
crystalline lens, but corresponding to the "parietal eye of other
crustaceans, and especially of the phyllopods, which is a lens-shaped
or pear-shaped sac, usually filled with sea water." He found that
above the "ocellus" the test was usually thin or even absent, and in a
few cases a dark line beneath seemed to outline the original form of
the sac. His summary follows:

     It is claimed that most, if not all, trilobites possessed a median
     or parietal eye on the glabella. [In proof of this assertion the
     following facts are stated:]

     1. A great number of species, belonging to more than thirty genera,
     possess a distinct tubercle on the glabella. This tubercle occurs
     alone in many genera, otherwise smooth, as in the Asaphidæ, and is
     hence of functional importance.

     2. In certain cases, as in _Cryptolithus tessellatus_, distinct
     lenticular bodies [not lenses] were recognized; in others, as in
     _Asaphus expansus_, only a thinner, probably transparent test.
     Many other species show a distinct pit in interior casts of the
     tubercle, indicating a lens-like thickening of the top of the
     tubercle. The median eye therefore probably possessed all the
     different stages of development seen in other crustaceans.

     3. As in the parietal eyes of the crustaceans and the eurypterids,
     the tubercles are most prominent and distinct in the earlier
     growth-stages, notably so in _Isotelus gigas_.

     4. The tubercle is especially well developed in the so-called blind
     forms where the lateral eyes are abortive, as in _Cryptolithus_
     (_Trinucleus_), _Dionide_, _Ampyx_.

     5. The tubercles always appear on the apex on the highest part of
     the glabella, where their visual function would be most useful.

     6. The tubercle is generally situated between the lateral eyes,
     like the parietal eye in crustaceans and eurypterids, on account of
     its close connection with the brain.

     7. Frequently it forms the posterior termination of a short crest,
     also as in certain eurypterids (_Stylonurus_), indicating the
     direction of the nerve.

     8. The median eye is borne on a tubercle or mound in the Ordovician
     and Silurian trilobites, while the tubercle is rarely noticed in
     the Devonian and in few Cambrian forms. In the Devonian forms,
     similarly as in many crustaceans and in later growth-stages of some
     asaphids, the strong development of the lateral eyes may have led
     to a loss of the parietal eyes. In the Cambrian genera evidence is
     present to suggest that the parietal eyes consisted only of
     transparent spots or lens-like thickenings of the exoskeleton,
     hardly noticeable from the outside.

     9. It is _a priori_ to be inferred that the trilobites should, as
     primitive crustaceans, have possessed median or parietal eyes.

As a student, I accepted Professor Beecher's dictum that this tubercle
represented a median _ocellus_, but more recently a number of things
have led me to the view that it is the point of attachment of the
ligament by which the heart is supported.

The chief arguments against its interpretation as a parietal eye seem
to be that its structure is not absolute proof, being capable of other
explanation; its position is variable, in front, between, or back of
the eyes; it is exactly like other tubercles on the median line,
especially the nuchal spine or tubercle, and the similar ones along
the axial lobe of the thorax; and it is not present in the protaspis
or very young trilobites.

1. The structure disclosed by Ruedemann's sections, a sort of sac-like
cavity beneath a thinned test, can be explained as a gland, a
ligamentary attachment, or a vestigial spine, as well as an eye. In a
section of _Asaphus expansus_, which I made some years ago when trying
to get some light on this problem, there is a similar cavity under the
pustule, but a secondary layer of shell lay beneath it and apparently
cut it off from the glabellar region, thus indicating that it had
lost its function in the adult of this animal. Sections through the
tubercles of the glabella of _Ceraurus_ show all of them hollow, with
very thin upper covering or none at all, and their structure is not
unlike that of the tubercle of _Cryptolithus_. In fact, sections can
be seen in Doctor Walcott's slices which are practically identical
with the one Ruedemann obtained from _Cryptolithus_. Since it is
obvious that not all of the pustules of a _Ceraurus_ could have been
eyes, the evidence from structure is rather against than for the
interpretation of the median pustule as such an organ.

2. The position of the tubercle varies greatly in different genera.
Where furthest forward (_Tretaspis_, _Goldius_), it is just back of
the frontal lobe, while in some species of asaphids it is in the neck
furrow. In species with compound eyes it is frequently between the
eyes, but more often back of them. If its history be traced in a
single family, it is generally found farthest forward in the more
ancient species and moves backward in the more recent ones. The eyes
do this same thing, but the median tubercle goes back further than the
eyes. This can be seen, for example, in the American Asaphidæ, where
the pustule is up between the eyes of _Hemigyraspis_ and _Symphysurus_
of the Beekmantown and back of the eyes of the _Isotelus_ of the
Trenton. Turning now to the under side of the head, it appears that
the tubercle bears a rather definite relation to the hypostoma. If the
hypostoma is short, the tubercle is well forward. If long, it is far
back on the head. It seems in many cases to be just back of the
posterior tip of the hypostoma, or just behind the position of the
mouth, while in others it is not as far back as the tip of the
hypostoma.

The median tubercle is in many cases developed into a long spine.
This is usually in an ancient member of a tubercle-bearing family,
and suggests that in most cases the tubercle is a vestigial organ.
An example of this occurs in _Trinucleoides_, the most ancient of the
Trinucleidæ. _Trinucleoides reussi_ (Barrande) (Supplement, 1872, pl.
5, figs. 17, 18) has a very long slender spine in this position. It
could be explained as an elevated median eye, but it also very
strongly suggests the zoæal spine of modern brachyuran Crustacea.
Gurney (Quart. Jour. Mic. Sci., vol. 46, 1902, p. 462) supports
Weldon in the conclusion that the long spines of the zoæa are
directive, and states that the animal swims in the direction of the
long axis of the spine. He also suggests that, since the period of
their presence corresponds to the period before the development of the
"auditory" organs, the spines may perform the functions of balancing
and orientation. It is generally admitted that the spine of the zoæa
is also protective, and the obvious function, first pointed out by
Spence Bate in 1859, is that it contains a ligament which helps
suspend the heart, which lies beneath the spine. This latter function
may have been that of the median tubercle in the trilobite. Such an
explanation would account for the backward migration mentioned above,
for as the stomach enlarged and the mouth moved backward on the
ventral side, the heart may have been pushed backward on the upper
side.

There is also a curious parallelism between the ontogenetic history of
the zoæal spine and the phylogenetic history of the Trinucleidæ or
Cheiruridæ (Nieszkowskia is the ancient member of this family in which
the spine replaces the tubercle). When first hatched, the larval crab
shows no trace of the spine, but very quickly it evaginates, lying
dorsally on the median line, pointing forward (Faxon, Bull. Mus. Comp.
Zool., vol. 6, 1880, pl. 2). With the splitting of the original
envelope, the spine becomes erect, but persists only a short time, and
is reduced to a vestigial tubercle toward the end of the zoæal stages,
its disappearance being, as pointed out by Gurney, coincident with the
development of the balancing organs. This manner of suspension of the
heart by a long tendon certainly does suggest that Gurney is right in
his interpretation of the function. Briefly, the zoæal spine served
for a short time a function later taken over by other organs. It was
not present in the youngest stages, it became prominent at a very
early stage, was soon vestigial, and then lost.

Take now the trilobites. There is no trace of the median pustule in
the protaspis of any form, and in many primitive trilobites it is
absent. It appears first as a long spine in certain families, and
later becomes vestigial and disappears. Very few trilobites of
Silurian and later times show it at all.

In the particular case of the Trinucleidæ, which were burrowers, the
spine is present on only the oldest and most primitive of the group, a
form which has only a most rudimentary fringe. It is obvious from the
large size of the pygidium in the larval trinucleid that this family
is derived from a group of free swimmers. _Trinucleoides reussi_ was
perhaps in the transitional stage, just leaving the swimming mode of
life, and belonged to a group which had not developed any other
"statocyst" than the median spine. Among the later Trinucleidæ the
spine became a vestigial tubercle, and in some cases entirely
disappeared. A similar history can be traced in the Cheiruridæ,
starting from some such forms as the American Lower Ordovician
_Nieszkowskia_ (_N. perforator_ p. ex.).

Another example of a median spine instead of a tubercle is in Goldius
rhinoceros (Barrande). Since this species is not from the oldest
Goldius-bearing rocks, but from the Lower Devonian, it does not follow
what seems to be the general rule, but makes an interesting exception.
Goldius rhinoceros (Barrande) (Supplement, 1872, pl. 9, figs. 12, 13)
has the median tubercle elevated into a stubby, recurved spine very
suggestive of the horn of a rhinoceros. Since the eyes of this species
are very well developed, there seems no especial reason for the
elevation of a parietal eye, and the example certainly does not
support that interpretation.

3. This tubercle is essentially similar to other tubercles on the
median line of cephalon, thorax, and even pygidium. This has been
discussed sufficiently under section 1 above, but it may perhaps be
justifiable to point out that in some of the Agnostidæ there is a
median tubercle on both shields, and since it has not yet been
demonstrated beyond question which shield is the cephalon, to say
which one is a parietal eye and which one is a tubercle is impossible.
In other words, the parietal eye can not be differentiated from any
other tubercle except by its position.

4. One of the as yet unexplained features of the protaspis of
trilobites is the absence of the "nauplius eye." Beecher (1897 B, p.
40) explained this on the ground of the extremely small size of the
protaspis and the imperfection of the preservation. If the median
tubercle were really a median eye, it should be present in the
protaspis and the earlier stages of the ontogeny, even if not in the
adult, and should certainly appear before the compound eyes. (In
_Limulus_, however, the compound eyes appear first.) The median eye
has not so far been seen in any young trilobite in any stage previous
to that in which compound eyes are present. The full ontogeny is not
known of any species with compound eyes in which the median tubercle
is present in the adult, but theoretically the median eye should be
most prominent in the young of just those primitive trilobites about
whose development most is known.

NERVOUS SYSTEM.

There has been a rather general impression among students of
trilobites that the eye-lines, which should be differentiated from the
genal cæca, denote the course of the optic nerves, but no other
evidence of the nervous system has been found, save the so-called
nervures which have been discussed above. In _Apus_ the nerves leading
to the eyes come off from the anterior ganglion or "brain" and run
directly to the eyes. If conditions were similar in the trilobites,
the "brain" was beneath the anterior glabellar lobe, provided, of
course, that the eye-lines do indicate the course of the optic nerve.

The ontogenetic history of the eye-lines of trilobites with compound
eyes is instructive, and has already been discussed by Lindstroem
(1901, pp. 12-25), but he did not cite the case of _Ptychoparia_,
which is particularly interesting, because in this genus both
eye-lines and "nervures" are present. Beecher (1895 C, p. 171, pl. 8,
figs. 5-7) has shown that in _Ptychoparia kingi_ the eye-lines of a
specimen in the metaprotaspis stage run forward at a low angle with
the glabella, while in the adult their course is nearly at right
angles to it. They have therefore swung through an arc of at least 60
and in so doing have had ample opportunity to become coincident with
the primary trunks of the genal cæca. Once that was accomplished, it
is quite likely that the one fold in the shell would continue to house
both structures. In other trilobites, there is a similar backward
progression of the eye-lines.

As would be expected, the ventral ganglia and the longitudinal cords
left no trace in the test. Since each segment has appendages, there
was probably a continuous chain of ganglia back to the posterior end
of the pygidium.

VARIOUS GLANDS.

_Dermal glands._--The surface of many trilobites is "ornamented" with
pustules and spines which on sectioning are nearly always found to be
hollow, and in many cases have a fine opening at the tip. While it is
generally believed that the purpose of these spines was protective,
yet it is possible that many of them were merely outgrowths which
increased the area through which the respiratory function could be
carried on. It will be recalled that most of the smooth trilobites
are punctate, some of them very conspicuously so, and the spines and
pustules of ornamented trilobites may merely subserve the same
function as the pores of smooth ones.

If the spines were protective, it would not be surprising if some of
them, hollow and open at the top, were poisonous also, and had glands
at the base. These are, however, purely matters of speculation so far.

_Renal excretory organs._--Nothing has been seen of any such organs,
unless the genal cæca may possibly be of that nature. The main trunks
of these always lead to the sides of the anterior glabellar lobe,
which is not the point of attachment of either antennæ or biramous
limbs, so that there seems little chance that they will bear this
interpretation.

_Reproductive organs._--Nothing is yet positively known about the
reproductive organs or the position of their external openings. If the
"exites" of _Neolenus_ could be interpreted as brood-pouches, which
does not seem probable, then the genital openings were located near
the base of some pair of anterior thoracic appendages.

_The Panderian Organs: Internal Gills or Poison Glands?_

At a meeting of the Mineralogical Society at St. Petersburg, Volborth
(1857) announced that Doctor Pander had two years before discovered
certain organs on the lower side of the doublure of the pleural lobes
of the thorax of a specimen of _Asaphus expansus_. These organs were
oval openings in the doublure, one near the posterior margin of the
cephalon, and one on each thoracic segment of the half-specimen
figured by Volborth in 1863. They were explained by Volborth and by
Eichwald (1860, 1863) as the points of attachment of appendages.
Billings (1870) described and figured the "Panderian organs" of
"_Asaphus platycephalus_" and stated that he had seen them in
_Asaphus_ [_Ogygites_] _canadensis_ and _A. megistos_ [_Isotelus
maximus_] as well. He thought some sort of organ was attached to them,
but could not suggest its function. Woodward (1870) thought that the
openings were "only the fulcral points on which the pleuræ move."
Their position outside the fulcra shows that this explanation is
impossible.

So far as I am aware, the Panderian organs have been seen only in
the Asaphidæ. Barrande figured them in "_Ogygia_" [_Hemigyraspis_]
_desiderata_ (1872) and Schmidt in two species of _Pseudasaphus_. They
seem to occupy the same position in Bohemian, Russian, and American
specimens. There is always one pair of openings on each thoracic
segment, and one pair in line with them on the posterior margin of the
cephalon. They occur near the anterior margin of the segment, and near
the inner end of the doublure. In some cases they are surrounded by a
ventrally projecting rim, while in others they have a thin edge. There
seem to be no markings on the interior of the shell which are
connected with them.

While thinking over the trilobites in connection with the origin of
insects, it occurred to me that these hitherto unexplained Panderian
organs might possibly be openings to internal gills and that the
Asaphidæ might have been tending toward an amphibious existence.
On mentioning this to Doctor R. V. Chamberlin of the Museum of
Comparative Zoology, he called my attention to the possibility that
they might be openings similar to those of the repugnatorial glands of
Diplopoda. While no definite decision as to the function can be made,
the explanation offered by Doctor Chamberlain seems more plausible
than my own, and has suggested still a third, namely, that they might
be the openings of poison glands.

If one were to argue that these apertures are really connected with
respiration, it might be pointed out that they are ventral in
position, while the _foramina repugnatoria_ are always dorsal or
lateral, even in diplopods with broad lateral expansions. If offensive
secretions were poured out beneath a concave shell like that of a
trilobite, they would be so confined as to be but slightly effective
against an enemy. This would indicate that if these openings were the
outlets of glands, the substance secreted might be a poison used to
render prey helpless. On the other hand, openings to gills are
normally ventral in position, and if the pleural lobes were folded
down against the body, they would be brought very close to the bases
of the legs.

A further curious circumstance is that so far no traces of exopodites
have been found on _Isotelus_. The endopodites of both _Isotelus
latus_ and _I. maximus_ are fairly well preserved in the single known
specimen of each, yet no authentic traces of exopodites have been
found with them. Moreover, Walcott sliced specimens of _Isotelus_ from
Trenton Falls and found only endopodites. It may also be recalled that
the finding of the specimen of _Isotelus arenicola_ at Britannia and
the tracks which I attributed to it, suggested to me that it was a
shore-loving animal (1910). It offers a field for further inquiry,
whether the Asaphidæ may not have had internal gills, and whether some
primitive member of the family may not have given rise to tracheate
arthropods.

[Illustration: Fig. 28. Side view of a specimen of _Isotelus gigas_
Dekay, from which the test of the pleural lobes has been broken to
show the position of the Panderian organs. Natural size. Specimen in
the Museum of Comparative Zoology.]

The explanation of the Panderian organs as openings of poison glands
is less radical than the one just set forth, and so possibly lies
nearer the truth. One would expect poison glands to lie at the bases
of fangs, and so they do in specialized animals like chilopods and
scorpions, but the trilobites may have had the less effective method
of pouring out the poison from numerous glands. The purpose may have
been merely to paralyze the brachiopod or pelecypod which was
incautious enough to open its shell in proximity to the asaphid.

MUSCULATURE.

This is a field which is rather one for investigation than for
exposition. Very little has been done, though probably much could be.
The chief obstacle to a clearer understanding of the muscular system
lies in the difficulty of getting at the inner surface of the test
without obscuring the faint impressions in the process.

There exist in the literature a number of references to scars of
attachment of muscles, and any study of the subject should of course
begin by the collection of such data. I shall at this time refer to
only a few observations on the subject.

The structure and known habits of trilobites make it obvious that
strong flexor and extensor muscles must have been present, and some
trace of them and of their points of attachment should be found. It is
likely that their proximal ends were tough tendons. The muscles
holding up the heart and alimentary canal would be less likely to
reveal their presence by scars, but there must have been at least one
pair of strong muscles extending from the under side of the head
across to the hypostoma. Judging from the method of attachment, the
muscles moving the limbs were short ones, chiefly within the segments
of the legs themselves.

_Flexor Muscles._

Since the majority of trilobites had the power of enrollment, and seem
also to have used the pygidia in swimming, the flexors must have been
important muscles. Beecher (1902, p. 170) appears to have been the
only writer to point out any tangible evidence of their former
presence. Walcott (1881, p. 199) had shown that the ventral membrane
was reinforced in each segment by a slightly thickened transverse
arch. Beecher showed that on this thickened arch in _Triarthrus_,
_Isotelus_, _Ptychoparia_, and _Calymene_, there are low longitudinal
internal ridges or folds. One of these is central, and there is a pair
of diagonal ridges on either side. Beecher interpreted these ridges as
separating the strands of the flexor muscles, and believed that a line
of median ridges divided a pair of longitudinal muscles, while the
outer ridges showed the place of attachment of the pair of strands
which was set off to each segment. He did not discuss the question as
to where the anterior and posterior ends were attached. In trilobites
with short pygidia, the attachment would probably have been near the
posterior end, and it is possible that the two scars beneath the
doublure and back of the last appendifers in _Ceraurus_ may indicate
the point of attachment in that genus.

There is as yet no satisfactory evidence as to where the anterior ends
of the flexors were attached. In _Apus_ these muscles unite in an
entosternal sinewy mass above the mouth, but no evidence of any
similar mass has been found in the trilobites and it is likely that
the muscles were anchored somewhere on the test of the head.

_Extensor Muscles._

The exact position of these muscles has not been previously discussed.
The interior of the dorsal test shows no such apodemes as are found on
the mesosternites, but, as I have shown in the discussion of the
alimentary canal of _Calymene_ and _Ceraurus_, there is an opening
on either side of the axial lobe between the dorsal test and the
abdominal sheath, and it is entirely probable that an extensor muscle
passed through each of these. The abdominal sheath extends only along
the posterior region of the glabella and the anterior part of the
thorax, and probably served to protect the soft organs from the strain
of the heavy muscles. The extensors (see fig. 29) probably lay along
the top of the axial lobe on either side of the median line of the
thorax to the pygidium, where they appear to have been attached
chiefly on the under side of the anterior ring of the axial lobe,
although strands probably continued further back. This is above and
slightly in front of the fulcral points on the pleura, and meets the
mechanical requirements. _Ceraurus_ (Walcott, 1875, and 1881, p. 222,
pl. 4, fig. 5) shows a pair of very distinct scars on the under side
of the first ring of the pygidium, and in many other trilobites
(_Illænus_, _Goldius_, etc.) distinct traces of muscular attachment
can be seen in this region, even from the exterior. The anterior ends
were probably attached by numerous small strands to the top of the
glabella, and, principally, to the neck-ring.

On enrolling, the sternites of all segments are pulled forward and the
tergites backward. In straightening out, the reverse process takes
place. The areas available for muscular attachment are so disposed as
to indicate longitudinal flexor and extensor muscles rather than short
muscles extending from segment to segment. Indeed, the tenuity of the
ventral membrane is such as to preclude the possibility of enrollment
by the use of muscles of that sort, while powerful longitudinal
flexors could have been anchored to cephalon and pygidium. The
strongly marked character of the neck-ring of trilobites is probably
to be explained as due to the attachment of the extensor muscles,
rather than to its recent incorporation in the cephalon. The same is
true of the anterior ring on the pygidium.

[Illustration: Fig. 29. Restoration of a part of the internal organs
of _Ceraurus pleurexanthemus_ as seen from above. At the sides are the
extensor muscles, and in the middle, the heart overlying the
alimentary canal. Drawn by Doctor Elvira Wood, under the supervision
of the author.]

_Possible preservations of extensors and flexors in Ceraurus_.--Among
Doctor Walcott's sections are four slices which I should not like to
use in proving the presence of longitudinal muscles, but which may be
admitted as corroborative evidence. Two of these transverse sections
(Nos. 114 and 199) show a dorsal and a ventral pair of dark spots in
positions which suggest that they represent the location of the dorsal
and ventral muscles, while two others (Nos. 131 and 140) show only the
upper pair of spots. The chief objection to this interpretation is
that it is difficult to imagine how the muscles could be so replaced
that they happen to show in the section. Both the sections showing all
four spots are evidently from the anterior part of the thorax, as they
show traces of the abdominal sheath, which seems to be squeezed
against the inside of the axial lobe, with the muscles (?) forced out
to the sides. The ventral pair lie just inside the appendifers, but
even if they are sections of muscles, all four are probably somewhat
out of place.

_Hypostomial Muscles._

The hypostoma fits tightly against the epistoma, or the doublure when
the epistoma is absent, but in no trilobite has it ever been seen
ankylosed to the dorsal test, and its rather frail connection
therewith is evidenced by the relative rarity of specimens found with
it in position. That the hypostoma was movable seems very probable,
and that it was held in place by muscles, certain. The maculæ were
always believed to be muscle scars until Lindstroem (1901, p. 8)
announced the discovery by Liljevall of small granules on those of
_Goldius polyactin_ (Angelin). These were interpreted as lenses
of eyes by Lindstroem, who tried to show that the maculæ of all
trilobites were functional or degenerate eyes. Most palæontologists
have not accepted this explanation, and since the so-called eyes cover
only a part of the surface of the maculæ, it is still possible to
consider the latter as chiefly muscle-scars.

In Lindstroem's summary (1901, pp. 71, 72) it is admitted that the
globular lenses are found only in _Bronteus_ (_Goldius_) (three
Swedish species only) and _Cheirurus spinulosus_ Nieszkowski, while
the prismatic structure supposed to represent degenerate eyes was
found in eleven genera (Asaphidæ, Illænidæ, Lichadidæ). All of these
are forms with well developed eyes, and Lindstroem himself points out
that the appearance of actual lenses in the hypostoma was a late
development, long after the necessity for them would appear to have
passed.

The use of the hypostoma has been discussed by Bernard (1892, p. 240)
and extracts from his remarks are quoted:

     The earliest crustacean-annelids possessed large labra or prostomia
     projecting backward, still retained in the Apodidæ and trilobites.
     This labrum almost necessitated a very deliberate manner of
     browsing. The animal would creep along, and would have to run some
     way over its food before it could get it into its mouth, the whole
     process, it seems to us, necessitating a number of small movements
     backwards and forwards. Small living prey would very often escape,
     owing to the fact that the animal's mouth and jaws were not ready
     in position for them when first perceived. The labrum necessitates
     the animal passing forwards over its prey, then darting backward to
     follow it with its jaws. We here see how useful the gnathobases of
     _Apus_ must be in catching and holding prey which had been thus
     passed over. Indeed the whole arrangement of the limbs of _Apus_
     with the sensory endites forms an excellent trap to catch prey
     over which the labrum has passed.

In alcoholic specimens of _Apus_ the labrum is not in a horizontal
plane, as it is in most well preserved trilobites, but is tipped down
at an angle of from 30 to 45, and the big mandibles lie under it. It
has considerable freedom of motion and is held in place by muscles
which run forward and join the under side of the head near its
posterior margin. It seems entirely possible that the hypostoma of
the trilobite had as much mobility as the labrum of _Apus_, and that
byopening downward it brought the mouth lower and nearer the food. It
will be recalled that the hypostomata of practically all trilobites
are pointed at the posterior margin, there being either a central
point or a pair of prongs. By dropping down the hypostoma until
the point or prongs rested on or in the substratum, and sending food
forward to the mouth by means of the appendages, a trilobite could
make of itself a most excellent trap, and if the animal could dart
backward as well as forward, the hypostoma would be still more useful.
There is no reason to suppose that they could not move backward, and
the "pygidial antennæ" of _Neolenus_ indicate that animals of that genus
at least did so. This habit of dropping down the hypostoma would also
permit the use of those anterior gnathobases which seem too far ahead
of the mouth in the trilobites with a long hypostoma.

For actual evidence on this point, it is necessary to have recourse
once more to Doctor Walcott's exceedingly valuable slices. From such
sections of _Ceraurus_ as his Nos. 100, 106, 108, 170, and 173, it is
evident that the hypostoma of that form could be dropped considerably
without disrupting the ventral membrane (fig. 30). Sections of
_Calymene_ already published (Walcott 1881, pl. 5, figs. 1, 2) show
the hypostoma turned somewhat downward, and the slices themselves show
sections of the anterior pair of gnathobases beneath the hypostoma.
When the hypostoma was horizontal, these gnathobases were crowded out
at the sides.

[Illustration: Fig. 30.--Longitudinal section of cephalon of _Ceraurus
pleurexanthemus_, to show position of the mouth and folds of the
ventral membrane between the glabella and the hypostoma. The test is
in solid black and the part within the ventral membrane dotted. From a
photographic enlargement. Specimen 169. × 3.9.]

[Illustration: Fig. 31.--A copy of Doctor Moberg's figure of _Nileus
armadillo_, showing the position of the muscle scars.]

If the hypostoma were used in the manner indicated, the muscles must
have been more efficient than those of the labrum of _Apus_, and it is
probable that they crossed to the dorsal test. Just where they were
attached is an unsolved problem. Barrande (1852, pl. 1, fig. 1) has
indicated an anterior pair of scars and a single median one on the
frontal lobe of _Dalmanites_ that may be considered in this connection,
and also three pairs of scars on the last two lobes of the glabella of
_Proëtus_ (1852, pl. 1, fig. 7). Moberg (1902, p. 295, pl. 3, figs. 2,
3, text fig. 1) has described in some detail the muscle-scars of a
rather remarkable specimen of _Nileus armadillo_ Dalman. While, as I
shall point out, I do not agree wholly with Professor Moberg's
interpretation, I give here a translation (made for Professor Beecher)
of his description, with a copy of his text figure:

     The well preserved surface of the shell permits one to note not
     only the tubercle (t) but a number of symmetrically arranged
     glabellar impressions. And because of their resemblance to the
     muscular insertions of recent crustaceans, I must interpret them as
     such. They appear partly as rounded hollows (k and i), also as
     elongate straight or curved areas (a, b, c, e, g, h) made up of
     shallow impressions or furrows about 1 mm. long, sub-parallel, and
     standing at an angle to the trend of the areas. Impression e is
     especially well marked, inasmuch as the perpendicular furrows are
     arranged in a shallow crescentic depression; and impression d shows
     besides the obscure furrows a number of irregularly rounded
     depressions. Larger similar ones occur at f, and in part extend
     over toward g.

     The meaning of these impressions, or their myologic significance,
     could be discussed, although such discussion might rather be termed
     guessing.

     Inner organs, such as the heart and stomach, might have been
     attached to the shell along impressions a and b. Also along or
     behind c and h, which both continue into the free cheeks, ligaments
     or muscular fibers may have been inserted. From d, e, f, and g,
     muscles have very likely gone out to the cephalic appendages.
     Against this it may be urged that impression d is too far forward
     to have belonged to the first pair of feet. Again, the impression h
     may in reality represent two confluent muscular insertions, from
     the first of which, in that case, arose the muscles of the fourth
     pair of cephalic feet. Were this the case, the muscles of the first
     pair of cheek feet should be attached at e. And d in turn may be
     explained as the attachment of the muscles of the antennæ, k those
     of the hypostoma, and from i possibly those of the epistoma. That k
     is here named as the starting point of the hypostomial muscles and
     not those of the antennæ, depends partly on the analogous position
     of i and partly on the fact that the hypostoma of _Nileus
     armadillo_ (text figure, in which the outline of the hypostoma is
     dotted), by reason of it? wing-like border, could not have
     permitted the antennæ to reach forward, but rather only outward or
     backward.

My own explanation would be that impressions e, f, and g correspond to
the glabellar furrows, h the neck furrow, and all four show the places
of attachment of the appendifers. Those at d may possibly be connected
with the antennæ, although I should expect those organs to be attached
under the dorsal furrows at the sides of the hypostoma. It will be
noted that either b, k, or i correspond well with the maculæ of the
hypostoma and some or all of them may be the points of attachment of
hypostomial muscles. They correspond also with the anterior scars of
_Dalmanites_.

EYES.

While I have nothing to add to what has been written about the eyes of
trilobites, this sketch of the anatomy would be incomplete without
some reference to the little which has been done on the structure of
these organs.

Quenstedt (1837, p. 339) appears to have been the first to compare the
eyes of trilobites with those of other Crustacea. Johannes Müller had
pointed out in 1829 (Meckel's Archiv) that two kinds of eyes were
found in the latter group, compound eyes with a smooth cornea, and
compound eyes with a facetted coat. Quenstedt cited _Trilobites
esmarkii_ Schlotheim (=_Illænus crassicauda_ Dalman) as an example of
the first group, and _Calymene macrophthalma_ Brongniart (=_Phacops
latifrons_ Bronn) for the second. Misreading the somewhat careless
style of Quenstedt, Barrande (1852, p. 133) reverses these, one of the
few slips to be found in the voluminous writings of that remarkable
savant.

Burmeister (1843; 1846, p. 19) considered the two kinds of eyes as
essentially the same, and accounted for the conspicuous lenses of
Phacops on the supposition that the cornea was thinner in that genus
than in the trilobites with smooth eyes.

Barrande (1852, p. 135) recognized three types of eyes in trilobites,
adding to Quenstedt's smooth and facetted compound eyes the groups of
simple eyes found in Harpes. In his sections of 1852, pl. 3, figs.
15-25, which are evidently diagrammatic, he shows separated biconvex
lenses in both types of compound eyes, _Phacops_ and _Dalmanites_ on
one hand, and _Asaphus_, _Goldius_, _Acidaspis_, and _Cyclopyge_
on the other. Clarke ( 1888), Exner ( 1891 ) and especially
Lindstroem (1901) have since published much more accurate figures and
descriptions. The first person to study the eye in thin section seems
to have been Packard (1880), who published some very sketchy figures
of specimens loaned him by Walcott. He studied the eyes of _Isotelus
gigas_, _Bathyurus longispinus_, _Calymene_, and _Phacops_, and
decided that the two types of eyes were fundamentally the same.
He also compared them with the eyes of _Limulus_.

Clarke (1888), in a careful study of the eye of _Phacops rana_, found
that the lenses were unequally biconvex, the curvature greater on the
inner surface. The lens had a circular opening on the inner side,
leading into a small pear-shaped cavity. The individual lenses were
quite distinct from one another, and separated by a continuation of
the test of the cheek.

Exner (1891, p. 34), in a comparison of the eyes of Phacops and
_Limulus_, came to the opinion that they were very unlike, and that
the former were really aggregates of simple eyes.

Lindstroem (1901, pp. 27-31) came to the conclusion that besides the
blind trilobites there were trilobites with two kinds of compound
eyes, trilobites with aggregate eyes, and trilobites with stemmata and
ocelli. His views may be briefly summarized.

     I. Compound eyes.

     1. Eyes with prismatic, plano-convex lenses.

     "A pellucid, smooth and glossy integument, a direct continuation of
     the common test of the body, covers the corneal lenses, quite as is
     the case in so many of the recent Crustacea. The lenses are closely
     packed, minute, usually hexagonal in outline, flat on the outer and
     convex on the inner surface. Such eyes are best developed in
     _Asaphus_, _Illænus_, _Nileus_, _Bumastus_, _Proëtus_, etc."

     2. Eyes with biconvex lenses.

     The surface of the eye is a mass of contiguous lenses, covered by a
     thin membrane which is frequently absent from the specimens, due to
     poor preservation. The lenses are biconvex, and being in contact
     with one another, are usually hexagonal, although in some cases
     they nearly retain their globular shape. Such eyes are found in
     Bury care, _Peltura_, _Sphæropthalmus_, _Ctenopyge_, _Goldius_,
     _Cheirurus_, and probably others.

     II. Aggregate eyes.

     The individual lenses are comparatively large, distinct from one
     another, each lying in its own socket. There is, however, a thin
     membrane, which covers all those in any one aggregate, and is a
     continuation of the general integument of the body. This membrane
     is continued as a thickened infolding which forms the sockets of
     the lenses.

     Such eyes are known in the Phacopidæ only.

     III. Stemmata and ocelli.

     The stemmata are present only in _Harpes_, where there may be on
     the summit of the cheek two or three ocelli lying near one another.
     Each, viewed from above, is nearly circular in outline, almost
     hemispheric, glossy and shining. In section they prove to be convex
     above and flat or slightly concave beneath. The test covers and
     separates them, as in the case of the aggregate eyes.

     The ocelli of the Trinucleidæ and _Eoharpes_ are smaller, and the
     detailed structure not yet investigated.

     Lindstroem concludes that so far as its facets or lenses are
     concerned, the eye of the trilobite shows the greatest analogy with
     the Isopoda, and the least with _Limulus_.

SUMMARY.

The simplest eyes found among the Trilobita are the ocelli. These
consist of a Simple thickening of the test to form a convex surface
capable of concentrating light. The similarity in position of the
paired ocelli of trilobites and the simple eyes of copepods has
perhaps a significance.

The schizochroal eyes may well be compared with the aggregate eyes of
the chilopods and scorpions. The mere presence of a common external
covering is not sufficient to prove this a true compound eye,
especially as the covering is merely a continuation of the general
test.

The holochroal eyes are of two kinds, one with plano-convex and one
with biconvex lenses. The latter would seem to be mechanically the
more perfect of the two, and it is worthy of note that the trilobites
possessing the biconvex lenses have, in general, much smaller eyes
than those with the other type.

If, as some investigators claim, the parietal eye of Crustacea
originates by the fusion of two lateral ocelli, trilobites show a
primitive condition in lacking this eye, which may have originated
through the migration toward the median line of ocelli like those of
the Trinucleidæ.

SEX.

That the sexes were separate in the Trilobita there can be very little
doubt, but the study of the appendages has as yet revealed nothing in
the way of sexual differences. One of the most important points still
to be determined is the location of the genital openings.

In many modern Crustacea, the antennæ or antennules are modified as
claspers, and it is barely possible that the curious double curvature
of the antennules of Triarthrus indicates a function of this sort. The
antennules of many specimens have the rather formal double curvature,
turning inward at the outer ends, and retain this position of the
frontal appendages, no matter what may be the condition of those on
the body. Other specimens have the antennules variously displaced,
indicating that they are quite flexible. It is conceivable that the
individuals with rigid antennules are males, the others females.

It is interesting to note that the antennules of _Ptychoparia
permulta_ Walcott (1918, pl. 21, fig. 1) have the same recurved form.
All the specimens of Neolenus, however, show very flexible antennas.

Barrande and Salter laid great stress upon the "forme longue" and
"forme large" as indicating male and female. This was based upon the
supposition that the female of any animal would probably have a
broader test than the male, a hypothesis which seems to be very little
supported by fact. In practical application it was found that the
apparent difference was so often due to the state of preservation or
the confusion of two or more species, that for many years little
reference has been made to this supposed sex difference.

EGGS.

In his classic work on the trilobites of Bohemia, Barrande described
three kinds of spherical and one of capsule-shaped bodies which he
considered to be the eggs of trilobites. After a review of the
literature and a study of specimens in the collections of the Museum
of Comparative Zoology, it can be said that none of these fossils has
proved to be a trilobite egg, but that they may be plants. A full
account of them will be published elsewhere.

Walcott (1881) and Billings (1870) have described similar bodies
within the tests of _Calymene_ and _Ceraurus_, but without showing
positive evidence as to their nature.

Methods Of Life.

This is a subject upon which much can be inferred, but little proved.
Without trying to cover all possibilities, it may be profitable to
see what can be deduced from what is known of the structure of the
external test, the internal anatomy, and the appendages. This can, to
a certain extent, be controlled by what is inferred from the strata
in which the specimens are found, the state of preservation, and the
associated animals. (For other details, see the discussion of
"Function of the Appendages" in Part I.)

HABITS OF LOCOMOTION.

The methods of locomotion may be deduced with some safety from a study
of the appendages, and, as has repeatedly been pointed out, all
trilobites could probably swim by their use. This swimming was
evidently done with the head directed forward, and could probably be
accomplished indifferently well with either the dorsal (gastronectic,
Dollo) or the ventral (notonectic) side up. If food were sought on the
bottom by means of sight, the animal would probably swim dorsal side
up, for by canting from side to side it could see the bottom just as
easily as though it were ventral side up, and at the same time it
would be in position to drop quickly on the prey. In collecting food
at the surface, it might swim ventral side up.

All trilobites could probably crawl by the use of the appendages, and,
as has already been pointed out, there are great differences in the
adjustment of the appendages to different methods of crawling. Some
crawled on their "toes," some by means of the entire endopodites, and
some apparently used the coxopodites to push themselves along. That
the normal direction of crawling was forward is indicated by the
position of the eyes and sensory antennules. There is no evidence that
their mechanism was irreversible, however, and the position of the
mouth and the shape of the hypostoma indicate that they usually backed
into feeding position. The caudal rami of Neolenus were evidently
sensory, and the animal was prepared to go in either direction.

The use of the pygidium as a swimming organ, suggested by Spencer
(1903, p. 492) on theoretical grounds, developed by Staff and Reck
(1911, p. 141) from a mechanical standpoint, and elaborated in
the present paper by evidence from the ontogeny, phylogeny, and
musculature, provided the animal with a swifter means of locomotion.
By a sudden flap of this large fin, a backward darting motion could be
obtained, which would be invaluable as a means of escape from enemies.
Staff and Reck seem to think that in this movement the two shields
were clapped together, and that the animal was projected along
with the hinge-like thorax forward. This might be a very plausible
explanation in the case of the bivalve-like Agnostidæ, and it is one
I had suggested tentatively for that family before I read Staff and
Reck's paper. In the case of the large trilobites with more segments,
however, it would be more natural to think of a mode of progression in
which there was an undulatory movement of the body and the pygidium,
up-and-down strokes being produced by alternately contracting the
dorsal and ventral muscles. Bending the pygidium down would tend to
pull the animal backward, while bringing it back into position would
push it forward. It follows, therefore, that one of these movements
must have been accomplished very quickly, the other slowly. If the
muscle scars have been interpreted properly, the ventral muscles were
probably the more powerful, an indication that the animal swam
backward, using the cephalon and antennules as rudders.

The chief objection to the theory of swimming by clapping the valves
together is that where the thorax consists of several segments it no
longer acts like the hinge of a bivalve, and a sudden downward flap of
the pygidium would impart a rotary motion to the animal. Take, for
example, such nearly spherical animals as the Illænidæ, and it will
readily be seen that there is nothing to give direction to the motion
if the pygidium be brought suddenly against the lower surface of the
cephalon. A lobster, it is true, progresses very well by this method,
but it depends upon its great claws and long antennæ to direct its
motions. The whole shape of the trilobite is of course awkward for a
rapidly swimming animal. It could keep afloat with the minimum of
effort and paddle itself about with ease, but it was not built on the
correct lines for speed.

Dollo (1910, p. 406), and quickly following his lead, Staff and Reck
(1911, p. 130), have published extremely suggestive papers, showing
that by the use of the principle of correlation of parts, much can be
inferred about the mode of life of the trilobites merely from the
structure of the test.

Dollo studied the connection between the shape of the pygidium and the
position and character of the eyes. As applied by him, and later by
Clarke and Ruedemann, to the eurypterids, this method seems most
satisfactory. He pointed out that in Eurypterida like _Stylonurus_ and
_Eurypterus_, where there is a long spine-like telson, the eyes are
back from the margin, so that a _Limulus_-like habit of pushing the
head into the sand by means of the limbs and telson was possible.
_Erettopterus_ and _Pterygotus_, on the other hand, have the eyes on
the margin, so that the head could not be pushed into the mud without
damage, and have a fin-like telson, suggesting a swimming mode of
life.

In carrying this principle over to the trilobites, Dollo was quite
successful, but Staff and Reck have pointed out some modifications
of his results. The conclusions reached in both these papers are
suggestive rather than final, for not all possible factors have been
considered. The following are given as examples of interesting
speculations along this line.

_Homalonotus delphinocephalus_, according to Dollo, was a crawling
animal adapted to benthonic life in the euphotic region, and an
occasional burrower in mud. This is shown by well developed eyes in
the middle of the cephalon, a pointed pygidium, and the plow-like
profile of the head. This was as far as Dollo went. From the very
broad axial lobe of _Homalonotus_ it is fair to infer that, like
_Isotelus_, it had very long, strong coxopodites which it probably
vised in locomotion, and also very well-developed longitudinal
muscles, to be used in swimming. From the phylogeny of the group, it
is known that the oldest homalonotids had broad unpointed pygidia of
the swimming type, and that the later species of the genus (Devonian)
are almost all found in sandstone and shale, and all have wider axial
lobes than the Ordovician forms. It is also known that the epistoma
is narrower and more firmly fused into the doublure in later than in
earlier species. These lines of evidence tend to confirm Dollo's
conclusion, but also indicate that the animals retained the ability to
swim well.

On the same grounds, _Olenellus thompsoni_ and _Dalmanites limulurus_
were assigned the same habitat and habits. Both were considered to
have used the terminal spine as does _Limulus_.

_Olenellus thompsoni_ is generally considered to be unique among
trilobites in having a _Limulus_-like telson in place of a pygidium.
This "telson" has exactly the position and characteristics of the
spine on the fifteenth segment of _Mesonacis_, and so long ago as
1896, Marr (Brit. Assoc. Adv. Sci., Rept. 66th Meeting, page 764)
wrote:

     The posterior segments of the remarkable trilobite _Mesonacis
     vermontana_ are of a much more delicate character than the anterior
     ones, and the resemblance of the spine on the fifteenth "body
     segment" of this species to the terminal spine of _Olenellus_
     proper, suggests that in the latter subgenus posterior segments of
     a purely membranous character may have existed devoid of hard
     parts.

This prophecy was fulfilled by the discovery of the specimens which
Walcott described as _Pædeumias transitans_, a species which is said
by its author to be a "form otherwise identical with _O. thompsoni_,
[but] has rudimentary thoracic segments and a _Holmia_-like pygidium
posterior to the fifteenth spine-bearing segment of the thorax." A
good specimen of this form was found at Georgia, Vermont, associated
with the ordinary specimens of _Olenellus thompsoni_, and I believe
that it is merely a complete specimen of that species. _Olenellus
gilberti_, which was formerly supposed to have a limuloid telson, has
now been shown by Walcott (Smithson. Misc. Coll., vol. 64, 1916, p.
406, pl. 45, fig. 3) to be a _Mesonacis_ and to have seven or eight
thoracic segments and a small plate-like pygidium back of the
spine-bearing fifteenth segment. All indications are that the spine
was not in any sense a pygidium. Walcott states that _Olenellus_
resulted from the resorption of the rudimentary segments of forms such
as _Mesonacis_ and _Pædeumias_, leaving the spine to function as a
pygidium. This would mean the cutting off of the anus and the
posterior part of the alimentary canal, and developing a new anal
opening on the spine of one of the thoracic segments!

If the spine of the fifteenth segment is not a pygidium, could it be
used, as Dollo postulates, as a pushing organ? Presumably not, for
though in entire specimens of _Olenellus_ (_Pædeumias_) it extends
back beyond the pygidium, it probably was borne erect, like the
similar spines in _Elliptocephala_, and not in the horizontal plane in
which it is found in crushed specimens.

While this removes some of the force of Dollo's argument, his
conclusion that _Olenellus_ was a crawling, burrowing animal living
in well lighted shallow waters was very likely correct. The long,
annelid-like body indicates numerous crawling legs, there is no
swimming pygidium, and the fusion of the cheeks in the head makes a
stiff cephalon well adapted for burrowing.

Staff and Reck have pointed out that _Dalmanites limulurus_ was not
entirely a crawler, but, as shown by the large pygidium, a swimmer
as well. This kind of trilobite probably represents the normal
development of the group in Ordovician and later times. The Phacopidæ,
Proëtidæ, Calymenidæ, and other trilobites of their structure could
probably crawl or swim equally well, and could escape enemies by
darting away or by "digging themselves in."

_Cryptolithus tessellatus_ (_Trinucleus concentricus_) is cited by
Dollo as an example of an adaptation to life in the aphotic benthos,
permanently buried in the mud. In this case he appealed to Beecher's
interpretation of the appendages, and pointed out that while the adult
is blind, the young have simple eyes and probably passed part of their
life in the lighted zone. It needs only a glance at the very young to
convince one that the embryos had swimming habits, so that in this
form one sees the adaptation of the individual during its history to
all modes of life open to a trilobite. The habits of the Harpedidæ may
have been similar to those of the Trinucleidæ, but the members of
this family are supplied with broad flat genal spines. It has been
suggested that these served like pontoons, runners, or snow-shoes, to
enable the animal to progress over soft mud without sinking into it.
Some such explanation might also be applied to the similar development
in the wholly unrelated Bathyuridæ. The absence of compound eyes and
the poor development of ocelli in the Harpedidæ suggest that they were
burrowers, and from these two families, Trinucleidæ and Harpedidæ, it
becomes evident that a pygidial point or spine is not a necessary part
of the equipment of a burrowing trilobite. In fact, from the habits of
_Limulus_ it is known that the appendages are relied upon for digging,
and that the telson is a useful but not indispensable pushing organ.

_Deiphon_ is an interesting trilobite from many points of view. Its
pleural lobes are reduced to a series of spines on either side of the
body, and its pygidium is a mere spinose vestige. Dollo considered
this animal a swimmer in the euphotic zone, because its eyes are on
the anterior margin, its body depressed, its glabella globose, and its
pygidium flat and spinose. That such a method of life was secondary
in a cheirurid was indicated to him by the fact that the more
primitive members of the family seemed adapted for crawling. Staff and
Reck have gone further and shown that the pygidium is only the vestige
of a swimming pygidium, and that the great development of spines
suggests a floating rather than a swimming mode of life. They
therefore argue for a planktonic habitat. A similar explanation is
suggested for _Acidaspis_ and other highly spinose species.

The Aeglinidæ, or Cyclopygidæ as they are more properly called,
present the most remarkable development of eyes among the trilobites.
In this, Dollo saw, as indeed earlier writers have, an adaptation
to a region of scanty light. The cephalon is not at all adapted to
burrowing, but the pygidium is a good swimming organ, and one is apt
to agree that this animal was normally an inhabitant of the ill
lighted dysphotic region, but also a nocturnal prowler, making trips
to the surface at night. Similar habits and habitat are certainly
indicated for _Telephus_ and the Remopleuridæ, but whether _Nileus_
and the large-eyed _Bumastus_ are capable of the same explanation is
doubtful.

Finch (1904, p. 181) makes the suggestion that "_Nileus_" (_Vogdesia_)
_vigilans_, an abundant trilobite in the calcareous shale of the
Maquoketa, was in the habit of burying itself, posterior end first. He
found a slab containing fifteen entire specimens, all of which had the
cephalon extended horizontally near the surface of the stratum, and
the thorax and pygidium projecting downward. The rock showed no
evidence that they were in burrows, and the fact that all were in the
same position indicates that the attitude was voluntarily assumed.
They appear to have entrenched themselves by the use of the pygidia,
which are incurved plates readily adapted for such use, and, buried up
to the eyes, awaited the coming of prey, but were, apparently,
smothered by a sudden influx of mud. The form of the eye in _Vogdesia
vigilans_ bears out this supposition of Finch's. Not only are the eyes
unusually tall, but the palpebral lobe is much reduced, so that many
of the lenses look upward and inward, as well as outward, forward and
backward. The particular food required by _V. vigilans_ must have been
very plentiful in the Maquoketa seas of Illinois and Iowa, for the
species was very abundant, but that its habits were self-destructive
is also shown by the great number of complete enrolled specimens of
all ages now found there. The soft mud was apparently fatal to the
species before the end of the Maquoketa, for specimens are seen but
very rarely in the higher beds.

_Vogdesia vigilans_ is shaped much like _Bumastus_, _Illænus_,
_Asaphus_, _Onchometopus_, and _Brachyaspis_, and it may be that these
trilobites with incurved pygidia had all adopted the habit of digging
in backward. As noted above, their pygidia are not very well adapted
for swimming, and most of them have large or tall eyes.

Dollo's comparison of the Cyclopygidæ to the huge-eyed modern amphipod
_Cystosoma_ is instructive. This latter crustacean, which has the
greater part of the dorsal surface of the carapace transformed into
eyes, is said to live in the dysphotic zone, at depths of from 40 to
100 fathoms, and to come to the surface at night. It swims ventral
side down.

The kinds of sediments in which trilobites are entombed have so far
afforded little evidence as to their habitat. Frech (Lethæa
palæozoica, 1897-1902, p. 67 _et seq._) who has collected such
evidence as is available on this subject, places as deeper water
Ordovician deposits the "Trinucleus-Schiefer" of the upper Ordovician
of northern Europe and Bohemia, the "Triarthrus-Schiefer" of America,
the "Asaphus-Schiefer" of Scandinavia, Bohemia, Portugal, and France,
and the Dalmania quartzite of Bohemia. .

_Cryptolithus_ and _Triarthrus_, although not confined to such
deposits, are apt to occur chiefly in very fine-grained shales, in
company with graptolites. These latter are distributed by currents
over great distances within short periods. It is somewhat curious that
the nearly blind burrowing Trinucleidæ, the dysphotic, large-eyed
Remopleuridæ and Telephus, the blind nektonic Agnostidæ and Dionide,
and the planktonic graptolites should go together and make up almost
the entire fauna of certain formations. Yet, when the life history of
each type is studied, a logical explanation is readily at hand, for
all have free-swimming larvæ.

A list of the methods of life noted above is given by way of summary,
with examples.

          {Planktonic  {Primarily    Earliest protaspis of all trilobites
          {            {Secondarily  _Deiphon_, _Odontopleura_, etc.
          {
  Pelagic {            {Primarily    Later protaspis of all trilobites.
          {            {               _Naraoia_
          {            {
          {            {            {Probably many thin-shelled
          {            {            {  trilobites with large pygidia
          {            {            {  (only partially nektonic)
          {Nektonic    {Secondarily {Cyclopygidæ    }
                                    {Remopleuridæ   } (nektonic dysphotic)

            {Crawlers and
            {  slow swimmers   Most trilobites with small pygidia.
            {                     _Triarthrus_, _Paradoxides_, etc.
  Benthonic {Crawlers and      Most trilobites with large pygidia.
            {  active swimmers   _Isotelus_, _Dalmanites_, etc.
            {
            {Crawlers, slow
            {  swimmers, and   Trinucleidæ, Harpedidæ,
            {  burrowers         some Mesonacidæ, etc.

FOOD AND FEEDING METHODS.

This subject has been less discussed than the methods of locomotion.
The study of the appendages has shown that while the mouth parts were
not especially powerful, they were at least numerous, and sufficiently
armed with spines to shred up such animal and vegetable substances as
they were liable to encounter. It having been ascertained that the
shape of the glabella and axial lobe furnishes an indication of the
degree of development of the alimentary canal it is possible to infer
something of the kind of food used by various trilobites.

The narrow glabellæ and axial lobes of the oldest trilobites would
seem to indicate a carnivorous habit, while the swollen glabellæ and
wider lobes of later ones probably denote an adaptation to a mixed or
even a vegetable diet. This can not be relied upon too strictly, of
course, for the swollen glabellæ of such genera as Deiphon or
Sphærexochus may be due merely to the shortening up of the cephalon.

Walcott (1918, p. 125) suggests that the trilobites lived largely upon
worms and conceives of them as working down into the mud and prowling
around in it in search of such prey. While there can be no doubt that
many trilobites had the power of burying themselves in loose sand or
mud, a common habit with modern crustaceans, most of them were of a
very awkward shape for habitual burrowers, and how an annelid could be
successfully pursued through such a medium by an animal of this sort
is difficult to understand. In fact, the presence of the large
hypostoma and the position of the mouth were the great handicaps of
the trilobite as a procurer of live animal food, and coupled with the
relatively slow means of locomotion, almost compel the conclusion that
errant animals of any size were fairly safe from it. This restricts
the range of animal food to small inactive creatures and the remains
of such larger forms as died from natural causes. The modern Crustacea
are effective scavengers, and it is probable that their early
Palæozoic ancestors were equally so. It is a common saying that in the
present stressful stage of the world's history, very few wild animals
die a natural death. In Cambrian times, competition for animal food
was less keen, and with the exception of a few cephalopods, a few
large annelids, and a few Crustacea like _Sidneyia_, there seem to
have been no aggressive carnivores. In consequence, millions of
animals must have daily died a natural death, and had there been no
way of disposing of their remains, the sea bottom would soon have
become so foul that no life could have existed. For the work of
removal of this decaying matter, the carnivorous annelids and the
Crustacea, mostly trilobites, were the only organisms, and it is
probable that the latter did their full share. After prowling about
and locating a carcass, the trilobite established himself over it, the
cephalon and hypostoma on one end and the pygidium on the other
enclosing and protecting the prey, which was shredded off and passed
to the mouth at leisure by means of the spinose endobases.

Even in Middle Cambrian times some trilobites (e. g., _Paradoxides_)
seem to have enlarged the capacity of the stomach and taken vegetable
matter, but later, in the Upper Cambrian and Ordovician, when the
development of cephalopods and fishes caused great competition for all
animal food, dead or alive, most trilobites seem to have become
omnivorous. This is of course shown by the swollen glabella, with
reduced lateral furrows, and, in the case of a few species
(_Calymene_, _Ceraurus_), the known enlargement of the stomach.

_Cryptolithus_ is the only trilobite which has furnished any actual
evidence as to its food. From the fact that the alimentary tract is
found stuffed from end to end with fine mud, and because it is known
to have been a burrower, it has been suggested by several that it was
a mud feeder, passing the mud through the digestive tract for the sake
of what organic matter it contained. Spencer (1903, p. 491) has
suggested a modification of this view:

     The phyllopods appear to feed by turning over whilst swimming and
     seizing with their more posterior appendages a little mud which
     swarms with infusoria, etc. This mud is then pushed along the
     ventral groove to the mouth. Casts, of the intestine of trilobites
     are still found filled with the mud.

_Ceraurus_ and _Calymene_ also must have occasionally swallowed mud in
quantity, otherwise the form of the alimentary canal could not have
been preserved as it is in a few of Doctor Walcott's specimens.

TRACKS AND TRAILS OF TRILOBITES.

Tracks and trails of various sorts have been ascribed by authors to
trilobites since these problematic markings first began to attract
attention, but as the appendages were until recently quite unknown,
all the earlier references were purely speculative. The subject is a
difficult one, and proof that any particular track or trail could have
been made in only one way is not easily obtained. Since the appendages
have actually been described, comparatively little has been done,
Walcott's work on _Protichnites_ (1912 B, p. 275) being the most
important. Since the first description of _Protichnites_ by Owen
(Quart. Jour. Geol. Soc., London, 1852, vol. 8, p. 247), it has been
thought that these trails were made by crustaceans, and the only known
contemporaneous crustaceans being trilobites, these animals were
naturally suggested. Dawson (Canadian Nat. Geol., vol. 7, 1862, p.
276) ascribed them, with reserve, to _Paradoxides_, and Billings
(1870, p. 484) suggested _Dikelocephalus_ or _Aglaspis_. Walcott
secured well preserved specimens which showed trifid tracks, and these
were readily explained when he found the legs of _Neolenus_, which
terminated with three large spines. Similar trifid terminations had
already been described by Beecher, and clearly pictured in his
restoration of _Triarthrus_, but the spines and the tracks had
somehow not previously been connected in the mind of any observer.
Walcott concluded that the tracks had been made by a species of
_Dikelocephalus_, possibly by _D. hartti_, which occurs both north
and south of the Adirondacks. In a recent paper, Burling (Amer. Jour.
Sci., ser. 4, vol. 44, 1917, p. 387) has argued that Protichnites was
not the trail of a trilobite, but of a "short, low-lying, more or less
heavy set, approximately 12-legged, crab-like animal," which had an
oval shape, toed in, and was either extremely flexible or else short
and more or less flexible in outline. This seems to describe a
trilobite.

_Climactichnites_, the most discussed single trail of all, has also
been ascribed to trilobites,--by Dana (Manual of Geology, 1863, p.
185), Billings (1870, p. 485), and Packard (Proc. Amer. Acad. Arts and
Sci., vol. 36, 1900, p. 64),--though less frequently than to other
animals. The latest opinion (see paper by Burling cited above) seems
to be against this theory.

Miller (1880, p. 217) described under the generic name
_Asaphoidichnus_ two kinds of tracks which were such as he supposed
might be made by an _Asaphus_ (_Isotelus_). In referring to the second
of the species, he says: "Some of the toe-tracks are more or less
fringed, which I attribute to the action of water, though Mr. Dyer is
impressed with the idea that it may indicate hairy or spinous feet."
The type of this species, _A. dyeri_, is in the Museum of Comparative
Zoology, and while it may be the trail of a trilobite, it would be
difficult to explain how it was produced.

Ringueberg (1886, p. 228) has described very briefly tracks found in
the upper part of the Medina at Lockport, New York. These consisted of
a regularly succeeding series of ten paired divergent indentations
arranged in two diverging rows, with the trail of the pygidium showing
between each series. The ten pairs of indentations he considered could
have been made by ten pairs of legs like those shown by the specimen
of Isotelus described by Mickleborough, and the intermittent
appearance of the impression of the pygidium suggested to him that the
trilobite proceeded by a series of leaps.

Walcott (1918, pp. 174-175, pl. 37-42) has recently figured a number
of interesting trails as those of trilobites, and has pointed out that
a large field remains open to anyone who has the patience to develop
this side of the subject.

PART III.

RELATIONSHIP OF THE TRILOBITES TO OTHER ARTHROPODA.

It can not be said that the new discoveries of appendagiferous
trilobites have added greatly to previous knowledge of the systematic
position of the group. Probably none will now deny that trilobites are
Crustacea, and more primitive and generalized than any other group in
that class. The chief interest at present lies in their relation to
the most nearly allied groups, and to the crustacean ancestor.

Trilobites have been most often compared with Branchiopoda, Isopoda,
and Merostomata, the present concensus of opinion inclining toward the
notostracan branchiopods (Apodidæ in particular) as the most closely
allied forms. It seems hardly worth while to burden these pages
with a history of opinion on this subject, since it was not until the
appendages were fully made out, from 1881 to 1895, that zoologists and
palæontologists were in a position to give an intelligent judgment.
The present status is due chiefly to Bernard (1894), Beecher (1897,
1900, et seq.), and Walcott (1912, et seq.).

The chief primitive characteristics of trilobites are: direct
development from a protaspis common to the subclass; variability in
the number of segments, position of the mouth, and type of eyes; and
serially similar biramous appendages.

The recent study has modified the last statement slightly, since it
appears that in some trilobites there was a modification of the
appendages about the mouth, suggesting the initiation of a set of
tagmata.

In comparing the trilobites with other Crustacea, the condition of the
appendages must be especially borne in mind, for while these organs
are those most intimately in contact with the environment, and most
subject to modification and change, yet they have proved of greatest
service in classification.

Appendages have been found on trilobites from only the Middle Cambrian
and Middle and Upper Ordovician, but as the Ordovician was the time of
maximum development of the group, it is probable that trilobites of
later ages would show degradational rather than progressive changes.
All the genera which are known show appendages of the same plan, and
although new discoveries will doubtless reveal many modifications of
that plan, general inferences may be drawn now with some assurance.

The chief characteristics of the appendages are: first, simple
antennules, a primitive feature in all Crustacea, as shown by
ontogeny; second, paired biramous appendages, similar to each other
all along the body, the youngest and simplest in front of the anal
segment, the oldest and most modified on the cephalon. The endobases
are retained on all the coxopodites, except possibly, in some species,
the anterior ones, and these gnathobases are modified in some genera
as mouth-parts, while in others they are similar throughout the
series. With these few fundamentals in mind, other Crustacea may be
examined for likenesses. The differences are obvious.

Crustacea.

BRANCHIOPODA.

The early idea that the trilobites were closely related to the
Branchiopoda was rejuvenated by the work of Bernard on the Apodidæ
(1892) and has since received the support of most writers on the
subject. Fundamentally, a great deal of the argument seems to be that
_Apus_ lies the nearest of any modern representative of the class to the
theoretical crustacean ancestor, and as the trilobites are the oldest
Crustacea, they must be closely related. Most writers state that the
trilobites could not be derived from the Branchiopoda (see, however,
Walcott 1912 A), nor the latter from any known trilobite, but both
subclasses are believed to be close to the parent stem.

Viewed from the dorsal side, there is very little similarity between
any of the branchiopods and the trilobites, and it is only in the
Notostraca, with their sessile eyes and depressed form, that any
comparison can be made. The chief way in which modern Branchiopoda and
Trilobita agree is that both have a variable number of segments in
the body, that number becoming very large in _Apus_ on the one hand and
_Mesonacis_ and _Pædeumias_ on the other. In neither are the appendages,
except those about the mouth, grouped in tagmata. Other likenesses
are: the Branchiopoda are the only Crustacea, other than Trilobita, in
which gnathobases are found on limbs far removed from the mouth; the
trunk limbs are essentially leaf-like in both, though the limb of the
branchiopod is not so primitive as that of the trilobite; caudal cerci
occur in both groups.

If the appendages be compared in a little more detail, the differences
prove more striking than the likenesses.

In the Branchiopoda, the antennules are either not segmented or only
obscurely so. In trilobites they are richly segmented.

In Branchiopoda, the antennæ are variable. In the Notostraca they are
vestigial, while in the males of the Anostraca they are powerful and
often complexly developed claspers. Either condition might develop
from the generalized biramous antennas of Trilobita, but the present
evidence indicates a tendency toward obsolescence. Claus' observations
indicate that the antennæ of the Anostraca are developments of the
exopodites, rather than of the endopodites.

The mandibles and maxillæ of the Branchiopoda are greatly reduced, and
grouped closely about the mouth. Only the coxopodites of the Trilobita
are modified as oral appendages.

The trunk limbs of _Apus_ are supposed to be the most primitive among
the Branchiopoda, and comparison will be made with them. Each
appendage consists of a flattened axial portion, from the inner margin
of which spring six endites, and from the outer, two large flat exites
(see fig. 34). This limb is not articulated with the ventral membrane,
but attached to it, and, if Lankester's interpretation of the origin
of schizopodal limbs be correct, then the limb of _Apus_ bears very
little relation to that of the Trilobita. In _Apus_ there is no
distinct coxopodite and the endobases which so greatly resemble the
similar organs in the Trilobita are not really homologous with them,
but are developments of the first endite. Beecher's comparison of the
posterior thoracic and pygidial limbs of _Triarthrus_ with those of
_Apus_ can not be sustained. Neither _Triarthrus_ nor any other
trilobite shows any trace of phyllopodan limbs. Beecher figured (1894
B, pl. 7, figs. 3, 4) a series of endopodites from the pygidium of a
young _Triarthrus_ beside a series of limbs from a larval _Apus_.
Superficially, they are strikingly alike, but while the endopodites of
_Triarthrus_ are segmented, the limbs of _Apus_ are not, and the parts
which appear to be similar are really not homologous. The similarity
of the thoracic limbs in the two groups is therefore a case of
parallelism and does not denote relationship.

Geologically, the Branchiopoda are as old as the Trilobita, and while
they did not have the development in the past that the trilobite
had, they were apparently differentiated fully as early. Anostraca,
Notostraca and Conchostraca, three of the four orders, are represented
in the Cambrian by forms which are, except in their appendages, as
highly organized as the existing species. Brief notes on the principal
Middle Cambrian Branchiopoda follow:

=Burgessia bella= Walcott.

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 177,
     pl. 27, figs. 1-3; pl. 30, figs. 3, 4.

This is the most strikingly like the modern Branchiopoda of any
species described by Walcott from the Middle Cambrian, and invites
comparison with _Apus_. The carapace is long, loosely attached to the
body, and extends over the greater part of the thorax. The eyes are
small, sessile, and close to the anterior margin.

The appendages of the head consist of two pairs of antennæ, and three
pairs of slender, jointed legs. Both pairs of antennæ are slender and
many-jointed, the antennules somewhat smaller than the antennæ. The
exact structure of the limbs about the mouth has not yet been made
out, but they are slender, tapering, endopodite-like legs, with at
least three or four segments in each, and probably more.

There are eight pairs of thoracic appendages, each limb having the
form of the endopodite of a trilobite and consisting of seven segments
and a terminal spine. The proximal three segments of each appendage
are larger than the outer ones, and have a flattened triangular
expansion on the inner side. Walcott also states that "One specimen
shows on seven pairs of legs, small, elongate, oval bodies attached
near the first joint to the outer side of the leg. These bodies left
but slight impression on the rock and are rarely seen. They appear to
represent the gills." They are not figured, but taken in connection
with the endopodite-like appearance of the segmented limbs, one would
expect them to be vestigial exopodites.

A small hypostoma is present on the ventral side, and several of the
specimens show wonderfully well the form of the alimentary canal and
the hepatic cæca. The main branches of the latter enter the mesenteron
just behind the fifth pair of cephalic appendages.

Behind the thorax the abdomen is long, limbless, and tapers to a
point. It is said to consist of at least thirty segments.

Compared with _Apus_, _Burgessia_ appears both more primitive and more
specialized. The carapace and limbless abdomen are _Apus_-like, but
there are very few appendagiferous segments, and the appendages are
not at all phyllopodan, but directly comparable with those of
trilobites, except, of course, for the uniramous character of the
cephalic limbs. A closer comparison may be made with _Marrella_.

=Waptia fieldensis= Walcott.

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 181,
     pl. 27, figs. 4, 5.

The carapace is short, covering the head and the anterior part of the
thorax. The latter consists of eight short segments with appendages,
while the six abdominal segments, which are similar to those of the
thorax, are without limbs except for the last, which bears a pair
of broad swimmerets. The eyes are marginal and pedunculate. The
antennules are imperfectly known, but apparently short, while the
antennas are long and slender, with relatively few, long, segments.
The mandibles appear to be like endopodites of trilobites and show
at least six segments. As so often happens in these specimens from
British Columbia, the preservation of the other appendages is
unsatisfactory. As illustrated (Walcott, 1912 A, pl. 27, fig. 5), both
endopodites and exopodites appear to be present, and the shaft of the
exopodite seems to be segmented as in _Triarthrus_.

Walcott considers _Waptia_ as a transitional form between the
Branchiopoda and the Malacostraca.

=Yohoia tenuis= Walcott.

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 172,
     pl. 29, figs. 7-13.

This species, though incompletely known, has several interesting
characteristics. The head shows, quite plainly in some specimens, the
five segments of which it is composed. The eyes are small, situated in
a niche between the first and second segments, and are described as
being pedunculate. The eight segments of the thorax all show short
triangular pleural extensions, somewhat like those of _Remopleurides_
or _Robergia_. The abdomen consists of four cylindrical segments, the
last with a pair of expanded caudal rami.

The antennules appear to be short, while the antennas are large, with
several segments, ending in three spines, and apparently adapted for
serving as claspers in the male. The third, fourth, and fifth pairs of
cephalic appendages are short, tapering, endopodite-like legs similar
to those of _Burgessia_.

The appendages of the thorax are not well preserved, and there seem to
be none on the abdomen.

This species is referred by Walcott to the Anostraca.

=Opabina regalis= Walcott.

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 167,
     pl. 27, fig. 6; pl. 28, fig. 1.

This most remarkably specialized anostracan is not well enough known
to allow comparison to be made with other contemporaneous Crustacea,
but it is worthy of mention.

There is no carapace, the eyes are pedunculated, thorax and abdomen
are not differentiated, and the telson is a broad, elongate, spatulate
plate. There seem to be sexual differences in the form of the anterior
cephalic and caudal appendages, but this is not fully established. The
most remarkable feature is the long, large, median cephalic appendage
which is so suggestive of the proboscis of the recent _Thamnocephalus
platyurus_ Packard. The appendages are not well enough preserved to
permit a determination as to whether they are schizopodal or
phyllopodan.

_Summary._

Walcott referred _Burgessia_ and _Waptia_ to new families under the
Notostraca, while _Yohoia_ and _Opabina_ were placed with the
Anostraca. Except for the development of the carapace, there is a
striking similarity between _Waptia_ and _Yohoia_, serving to connect
the two groups.

The Branchiopoda were very highly specialized as early as Middle
Cambrian time, the carapace of the Notostraca being fully developed
and the abdomen limbless. Some (_Burgessia_) had numerous segments,
but most had relatively few. The most striking point about them,
however, is that so far as is known none of them had phyllopodan
limbs. While the preservation is in most cases unsatisfactory, such
limbs as are preserved are trilobite-like, and in the case of
_Burgessia_ there can be no possible doubt of the structure. Another
interesting feature is the retention by _Yohoia_ of vestiges of
pleural lobes. The Middle Cambrian Branchiopoda are more closely
allied to the Trilobita than are the modern ones, but still the
subclass is not so closely related to that group as has been thought.
Modern _Apus_ is certainly much less like a trilobite than has been
supposed, and very far from being primitive. The Branchiopoda of the
Middle Cambrian could have been derived from the trilobites by the
loss of the pleural lobes, the development of the posterior margin of
the cephalon to form a carapace, and the loss of the appendages from
the abdominal segments. Modern branchiopods can be derived from those
of the Middle Cambrian by the modification of the appendages through
the reduction of the endopodite and exopodite and the growth of the
endites and exites from the proximal segments.

Carpenter (1903, p. 334), from his study of recent crustaceans, has
already come to the conclusion that the Branchiopoda are not the most
primitive subclass, and this opinion is strengthened by evidence
derived from the Trilobita and from the Branchiopoda of the Middle
Cambrian.

COPEPODA.

The non-parasitic Eucopepoda are in many ways much nearer to the
trilobites than any other Crustacea. These little animals lack the
carapace, and the body is short, with typically ten free segments and
a telson bearing caudal furcæ. The head is composed of five segments
(if the first thoracic segment is really the fused first and second),
is often flattened, and lacks compound eyes. Pleural lobes are well
developed, but instead of being flattened as in the trilobite, they
are turned down at the sides or even incurved. A labrum is present.

The antennules, antennæ, and mandibles are quite like those of
trilobites. The antennules are very long and made up of numerous
segments. The antennæ are biramous, the junction between the
coxopodite and basipodite is well marked, and the endopodite consists
of only two segments.

The mandibles are said to "retain more completely than in any other
Crustacea the form of biramous swimming limbs which they possess in
the nauplius." The coxopodites form jaws, while both the reduced
endopodite and exopodite are furnished with long setæ. The maxillulæ
are also biramous, but very different in form from those of the
trilobite, and the maxillæ are phyllopodan.

The first thoracic limb is uniramous and similar to the maxillæ, but
the five following pairs are biramous swimming legs with coxopodite,
basipodite, exopodite, and endopodite. Both the exopodite and
endopodite are shorter than in the trilobites, but bear setæ and
spines.

The last pair of thoracic limbs are usually modified in the male into
copulatory organs. In some females they are enlarged to form plates
for the protection of the eggs, in others they are unmodified. In
still others they are much reduced or disappear. The abdomen is
without appendages.

The development in Copepoda is direct, by the addition posteriorly to
the larval form (nauplius) of segments, and the appendages remain
nearly unmodified in the adult.

Altogether, the primitive Copepoda seem much more closely allied to
the Trilobita than any other modern Crustacea, but unfortunately no
fossil representative of the subclass has been found. This is not so
surprising when one considers the habits and the habitat of most of
the existing species. Many are parasitic, many pelagic in both fresh
and marine waters, and many of those living on the bottom belong to
the deep sea or fresh water. Most free-living forms are minute, and
all have thin tests.

The eyes of copepods are of interest, in that they suggest the paired
ocelli of the Harpedidæ and Trinucleidæ. In the Copepoda there are, in
the simplest and typical form of these organs, three ocelli, each
supplied with its own nerve from the brain. Two of these are dorsal
and look upward, while the third is ventral. In some forms the dorsal
ocelli are doubled, so that five in all are present (cf. some species
of Harpes with three ocelli on each mound). In some, the cuticle over
the dorsal eyes is thickened so as to form a lens, as appears to be
the case in the trilobites. These peculiar eyes may be a direct
inheritance from the Hypoparia.

ARCHICOPEPODA.

Professor Schuchert has called my attention to the exceedingly curious
little crustacean which Handlirsch (1914) has described from the
Triassic of the Vosges. Handlirsch erected a new species, genus,
family, and order for this animal, which he considered most closely
allied to the copepods, hence the ordinal name. _Euthycarcinus
kessleri_, the species in question, was found in a clayey lens in the
Voltzia sandstone (Upper Bunter). Associated with the new crustacean
were specimens of _Estheria_ only, but in the Voltzia sandstone itself
land plants, fresh and brackish water animals, and occasionally,
marine animals are found. The clayey lens seems to have been of fresh
or brackish water origin.

All of the specimens (three were found) are small, about 35 mm. long
without including the caudal rami, crushed flat, and not very well
preserved. The head is short, not so wide as the succeeding segments,
and apparently has large compound eyes at the posterior lateral
angles. The thorax consists of six segments which are broader than the
head or abdomen. The abdomen, which is not quite complete in any one
specimen, is interpreted by Handlirsch as having four segments in the
female and five in the male. Least satisfactory of all are traces of
what are interpreted by the describer as a pair of long stiff
unsegmented cerci or stylets on the last segment.

The ventral side of one head shield shows faint traces of several
appendages which must have presented great difficulty in their
interpretation. A pair of antennules appear to spring from near the
front of the lower surface, and the remainder of the organs are
grouped about the mouth, which is on the median line back of the
center. Handlirsch sees in these somewhat obscure appendages four
pairs of biramous limbs, antennæ, mandibles, maxillulæ, and maxillæ,
both branches of each consisting of short similar segments,
endopodites and exopodites being alike pediform.

Each segment of the thorax has a pair of appendages, and those on
the first two are clearly biramous. The endopodites are walking legs
made up of numerous short segments (twelve or thirteen according to
Handlirsch's drawing), while the exopodite is a long breathing and
rowing limb, evidently of great flexibility and curiously like the
antennules of the same animal. The individual segments are narrow at
the proximal end, expand greatly at the sides, and have a concave
distal profile. A limb reminds one of a stipe of _Diplograptus_.
Both branches are spiniferous.

No appendages are actually present on the abdomen, but each segment
has a pair of scars showing the points of attachment. From the small
size of these, it is inferred that the limbs were poorly developed.

This species is described in so much detail because, if it is a
primitive copepod, it has a very important bearing on the ancestry of
that group and is the only related form that has been found fossil.

The non-parasitic copepods have typically ten (eleven) free segments,
including the telson, and the four abdominal segments are much more
slender than the six in front of them. In this respect the agreement
is striking, and the presence of five pairs of appendages in the head
and six free segments in the thorax is a more primitive condition than
in modern forms where the first two thoracic segments are apparently
fused (Calman, 1909, p. 73).

The large compound eyes of this animal are of course not present in
the copepods, but as vestiges of eyes have been found in the young of
_Calanus_, it is possible that the ancestral forms had eyes.

The greatest difficulty is in finding a satisfactory explanation of
the appendages. The general condition is somewhat more primitive than
in the copepods, for all the appendages are biramous, while in the
modern forms the maxillipeds are uniramous and the sixth pair of
thoracic appendages are usually modified in the male as copulatory
organs. In the copepods the modification is in the direction of
reduction, both endopodites and exopodites usually possessing fewer
segments than the corresponding branches in the trilobites. The
endopodite of _Euthycarcinus_, on the contrary, possesses, if
Handlirsch's interpretation is correct, twice as many segments as the
endopodite of a trilobite. If the Copepoda are descended from the
trilobites, as everything tends to indicate, then _Euthycarcinus_ is
certainly not a connecting link. The only truly copepodan
characteristic of this genus is the agreement in number and
disposition of free segments. The division into three regions instead
of two, the compound eyes, and the structure of the appendages are all
foreign to that group.

With the Limulava fresh in mind, one is tempted to compare
_Euthycarcinus_ with that ancient type. The short head and large
marginal eyes recall _Sidneyia_, and the grouping of the appendages
about the mouth also suggests that genus and _Emeraldella_. In the
Limulava likewise there is a contraction of the posterior segments,
although it is behind the ninth instead of the sixth. There is no
likeness in detail between the appendages of the Limulava and those of
_Euthycarcinus_, but the composite claws of _Sidneyia_ show that in
this group there was a tendency toward the formation of extra
segments.

If this fossil had been found in the Cambrian instead of the Triassic,
it would probably have been referred to the Limulava, and is not
at all impossible that it is a descendant from that group. As a
connecting link between the Trilobita and Copepoda it is, however,
quite unsatisfactory.

OSTRACODA.

The bivalved shell of the Ostracoda gives to this group of animals an
external appearance very different from that of the trilobites, but
the few appendages, though highly modified, are directly comparable.
The development, although modified by the early appearance of the
bivalved shell within which the nauplius lies, is direct. Imperfect
compound eyes are present in one family.

The antennules are short and much modified by functioning as swimming,
creeping, or digging organs. They consist of eight or less segments.
The antennas are also locomotor organs, and in most orders are
biramous. The mandibles are biramous and usually with, but sometimes
without, a gnathobase. The maxillulæ are likewise biramous but much
modified.

The homology of the third post-oral limb is in question, some
considering it a maxilla and others a maxilliped. It has various forms
in different genera. It is always much modified, but exopodite and
endopodite are generally represented at least by rudiments. The fourth
post-oral limb is a lobed plate, usually not distinctly segmented, and
the fifth a uniramous pediform leg. The sixth, if present at all, is
vestigial.

Very little comparison can be made between the Ostracoda and
Trilobita, other than in the ground-plan of the limbs, but the
presence of biramous antennæ is a primitive characteristic.

CIRRIPEDIA.

Like the ostracod, the adult cirriped bears little external
resemblance to the trilobite. The form of the nauplius is somewhat
peculiar, but it has the typical three pairs of appendages, to which
are added in the later metanauplius stages the maxillæ and six pairs
of thoracic appendages. In the adult, the antennules, which serve for
attachment of the larva, usually persist in a functionless condition,
while the antennas disappear. The mandibles, maxillulæ, and maxillæ
are simple and much modified to form mouth parts, and the six pairs of
thoracic appendages are developed into long, multisegmented, biramous
appendages bearing numerous setæ which serve for catching prey. Paired
eyes are present in later metanauplius stages, but lost early in the
development. The relationship to the trilobite evidently is not close.

MALACOSTRACA.

_1. Phyllocarida._

The oldest malacostracans whose appendages are known are species of
_Hymenocaris_. One, described as long ago as 1866 by Salter, has what
seem to be a pair of antennæ and a pair of jaw-like mouth-parts.
Another more completely known species has recently been reported by
Walcott (1912 A, p. 183, pl. 31, figs. 1-6). This latter form is
described as having five pairs of cephalic appendages: a pair of
minute antennules beside the small pedunculated eyes, a pair of large
uniramous antennæ, slender mandibles and maxillulæ, and large maxillæ
composed of short stout segments. There are eight pairs of biramous
thoracic limbs, the exopodites setiferous, the endopodites composed of
short wide segments and ending in terminal claw-like spines. These
appendages are like those of trilobites.

_Hymcnocaris_ belongs to the great group of extinct ceratocarid
Crustacea which are admitted to the lowest of the malacostracan
orders, Phyllocarida, because of their resemblance to _Nebalia_,
_Paranebalia_, _Nebaliopsis_, and _Nebaliella_, the four genera which
are at present living. The general form of the recent and fossil
representatives of the order is strikingly similar. The chief outward
difference is that in many of the fossils the telson is accompanied by
two furcal rami, while in the modern genera it is simple. It now
becomes possible to make some comparison between the appendages of
_Hymcnocaris_ of the Middle Cambrian and the Nebaliidæ of modern seas.

In both there are five pairs of cephalic and eight of thoracic
appendages, while those of the abdomen of Hymenocaris are not known.

In both, the antennules are less developed than the antennæ. In the
Nebaliidæ the antennules show evidence of having been originally
double (they are obviously so in the embryo), while they are single in
_Hymcnocaris_. In both, the antennæ are simple. The remaining cephalic
organs are too little shown by the specimen from the Middle Cambrian
to allow detailed comparison. The mandibles, maxillulæ, and maxillæ of
_Nebalia_ are, however, of types which could be derived from the
trilobite.

In three of the genera of the Nebaliidæ, the eight pairs of thoracic
limbs are all similar to one another, though those of the genera
differ. All are biramous. The limbs of _Hymcnocaris_ can apparently be
most closely correlated with those of _Nebalia antarctica_, in which
the endopodite consists of short flattened segments, and the exopodite
is a long setiferous plate. Epipodites are present in both _Nebalia_
and _Hymcnocaris_.

So far as the appendages of _Hymenocaris_ are known, they agree very
well with those of the Nebaliidæ, and since they are of the trilobite
type, it may safely be stated that the Trilobita and Malacostraca are
closely related.

_2. Syncarida._

Walcott (1918, p. 170) has compared the limbs of _Neolenus_ with those
of the syncarid genera _Anaspides_ and _Koonunga_. These are primitive
Malacostraca without a carapace, but as they have a compressed test
and _Anaspides_ has stalked eyes, their gross anatomy does not suggest
the trilobite. The thoracic appendages are very trilobite-like, since
the endopodite has six segments (in _Anaspides_) and a multisegmented
setiferous exopodite. The coxopodites, except of the first thoracic
segment, do not, however, show endobases, and those which are
present are peculiar articulated ones. The cephalic appendages are
specialized, and the antennules double as in most of the Malacostraca.
External epipodites are very numerous on the anterior limbs.

This group extends back as far as the Pennsylvanian and had then
probably already become adapted to fresh-water life. It may be
significant that the Palæozoic syncarids appear to have lacked
epipodites. While differing very considerably from the Trilobita, the
Syncarida could have been derived from them.

_3. Isopoda._

Since the earliest times there has been a constant temptation to
compare the depressed shields of the trilobites with the similar ones
of isopods. Indeed, when _Scrolls_ with its Lichadian body was first
discovered about a hundred years ago, it was thought that living
trilobites had been found at last. The trilobate body, cephalic
shield, sessile eyes, abdominal shield, and pleural extensions make a
wonderful parallel. This similarity is, however, somewhat superficial.
The appendages are very definitely segregated in groups on the various
regions of the body, and while the pleopods are biramous, the thoracic
legs are without exopodites (except in very early stages of
development of one genus). The Isopoda arose just at the time of the
disappearance of the Trilobita, and there seems a possibility of a
direct derivation of the one group from the other. It should be
pointed out that while the differences of Isopoda from Trilobita are
important, they are all of a kind which could have been produced by
the development from a trilobite-like stock. For example:

Isopoda have a definite number of segments. There is less variation in
the number of segments among the later than the earlier trilobites.

Isopoda have no facial suture. In at least three genera of trilobites
the cheeks become fused to the cranidium and the sutures obliterated.

Isopoda have one or two segments of the thorax annexed to the head.
While this is not known to occur in trilobites, it is possible that it
did.

Most Isopoda have a fairly stiff ventral test. The ventral membrane of
trilobites would probably have become stiffened by impregnation of
lime if the habit of enrollment had been given up.

In Isopoda the antennæ are practically uniramous sensory organs. The
second cephalic appendages of trilobites are capable of such
development through reduction of the exopodite.

In the Isopoda the coxopodites are usually fused with the body,
remaining as free, movably articulated segments only in a part of the
thoracic legs of one suborder, the Asellota. Endobases are entirely
absent. This is of course entirely unlike the condition in Trilobita,
but a probable modification.

In Isopoda there is a distinct grouping of the appendages, with
specialization of function. The trilobites show a beginning of
tagmata, and such development would be expected if evolution were
progressive.

In both groups, development from the embryo is direct. Rudiments of
exopodites of thoracic legs have been seen in the young of one genus.

The oldest known isopod is _Oxyuropoda ligioides_ Carpenter and
Swain (Proc. Royal Irish Acad., vol. 27, sect. B, 1908, p. 63,
fig. 1), found in the Upper Devonian of County Kilkenny, Ireland. The
appendages are not known, but the test is in some ways like that of a
trilobite. The thorax, abdomen, and pygidium are especially like those
of certain trilobites, and there is no greater differentiation between
thorax and abdomen than there is between the regions before and behind
the fifteenth segment of a _Pædeumias_ or _Mesonacis_. The anal
segment is directly comparable to the pygidium of a _Ceraurus_, the
stiff unsegmented uropods being like the great lateral spines of that
genus.

The interpretation of the head offered by Carpenter and Swain is very
difficult to understand, as their description and figure do not seem
to agree. What they consider the first thoracic segment (fused with
the head) seems to me to be the posterior part of the cephalon and it
shows at the back a narrow transverse area which is at least analogous
to the nuchal segment of the trilobite. If this interpretation can be
sustained, _Oxyuropoda_ would be a very primitive isopod in which the
first thoracic segment (second of Carpenter and Swain) is still free.
According to the interpretation of the original authors, the species
is more specialized than recent Isopoda, as they claim that two
thoracic segments are fused in the head. The second interpretation was
perhaps made on the basis of the number of segments (nineteen) in a
recent isopod.

=Marrella splendens= WALCOTT.

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 192,
     pls. 25, 26.

Among the most wonderful of the specimens described by Doctor
Walcott is the "lace crab." While the systematic position was not
satisfactorily determined by the describer, it has been aptly compared
to a trilobite. The great nuchal and genal spines and the large
marginal sessile eyes, coupled with the almost total lack of thoracic
and abdominal test, give it a bizarre appearance which may obscure its
real relationships.

The cephalon appears to bear five pairs of appendages, antennules, and
antennæ, both tactile organs with numerous short segments, mandibles,
and first and second maxillæ. The last three pairs are elongate, very
spinose limbs, of peculiar appearance. They seem to have seven
segments, but are not well preserved. These organs are attached near
the posterior end of the labrum.

There are twenty-four pairs of biramous thoracic appendages, which
lack endobases. The endopodites are long and slender, with numerous
spines; the exopodites have narrow, thin shafts, with long, forward
pointed setæ. The anal segment consists of a single plate.

Further information about this fossil will be eagerly awaited. None of
the illustrations so far published shows biramous appendages on the
cephalon. This, coupled with the presence of tactile antennæ, makes
its reference to the Trilobita impossible, but the present
interpretation indicates that it was closely allied to them.

[Illustration: Fig. 32. _Marrella splendens_ Walcott. Restoration of
the ventral surface, based upon the photographs and descriptions
published by Walcott. Although all the limbs of the trunk appear to be
biramous, only endopodites are placed on one side and exopodites on
the other, for the sake of greater clearness in the illustration.
Drawn by Doctor Elvira Wood, under the supervision of the writer.
× about 6.]

_Restoration of Marrella._

(Text fig. 32.)

The accompanying restoration of the ventral surface of _Marrella_ is a
tentative one, based on Doctor Walcott's description and figures. The
outline is taken from his plate 26, figure 1; the appendages of the
head from plate 26, figures 1-3, 5, and plate 25, figures 2, 3; the
endopodites, shown on the left side only, from figures 3 and 6, plate
25. I have not studied actual specimens, and the original description
is very incomplete. The restoration is therefore subject to revision
as the species becomes better known.

Arachnida.

No attempt will be made to pass in review all of the subclasses of the
arachnids. Some of the Merostomata are so obviously trilobite-like
that it would seem that their relationship could easily be proved. The
task has not yet been satisfactorily accomplished, however, and new
information seems only to add to the difficulties.

So far as I know, the Araneæ have not previously been compared
directly with trilobites, although such treatment consists merely in
calling attention to their crustacean affinities, as has often been
done.

Carpenter's excellent summary (1903, p. 347) of the relationship of
the Arachnida to the trilobites may well be quoted at this point:

     The discussion in a former section of this essay on the
     relationship between the various orders of Arachnida led to the
     conclusion that the primitive arachnids were aquatic animals,
     breathing by means of appendicular gills. Naturally, therefore, we
     compare the arachnids with the Crustacea rather than with the
     Insecta. The immediate progenitors of the Arachnida appear to have
     possessed a head with four pairs of limbs, a thorax with three
     segments, and an abdomen with thirteen segments and' a telson, only
     six of which can be clearly shown by comparative morphology to have
     carried appendicular gills. But embryological evidence enables us
     to postulate with confidence still more remote ancestors in which
     the head carried well developed compound eyes and five pairs of
     appendages, while it may be supposed that all the abdominal
     segments, except the anal, bore limbs. In these very ancient
     arthropods, all the limbs, except the feelers, had ambulatory and
     branchial branches; and one important feature in the evolution of
     the Arachnida must have been the division of labour between the
     anterior and posterior limbs, the former becoming specialized for
     locomotion, the latter for breathing. Another was the loss of
     feelers and the degeneration of the compound eyes. Thus we are led
     to trace the Arachnida (including the Merostomata and Xiphosura)
     back to ancestors which can not be regarded as arachnids, but which
     were identical with the primitive trilobites, and near the
     ancestral stock of the whole crustacean class.

TRILOBITES NOT ARACHNIDA.

While no one having any real knowledge of the Trilobita has adopted
Lankester's scheme of the inclusion of the group as the primitive
grade in the Arachnida, reference to it may not be amiss. This theory
is best set forth in the Encyclopædia Britannica, Eleventh Edition,
under the article on Arachnida. It is there pointed out that the
primitive arachnid, like the primitive crustacean, should be an animal
without a fixed number of somites, and without definitely grouped
tagmata. As Lankester words it, they should be anomomeristic and
anomotagmatic. The trilobites are such animals, and he considers them
Arachnida and not Crustacea for the following reasons:

Firstly and chiefly, because they have only one pair (apart from the
eyes) of pre-oral appendages. "This fact renders their association
with the Crustacea impossible, if classification is to be the
expression of genetic affinity inferred from structural coincidence."

Secondly, the lateral eyes resemble no known eyes so closely as the
lateral eyes of _Limulus_.

Thirdly, the trilobation of the head and body, due to the expansion
and flattening of the sides or pleura, is like that of _Limulus_, but
"no crustacean exhibits this trilobite form."

Fourthly, there is a tendency to form a pygidial or telsonic shield,
"a fusion of the posterior somites of the body, which is precisely
identical in character with the metasomatic carapace of _Limulus_." No
crustacean shows metasomatic fusion of segments.

Fifthly, a large post-anal spine is developed "in some trilobites" (he
refers to a figure of _Dalmanites_).

Sixthly, there are frequently lateral spines on the pleura as in
_Limulus_. No crustacean has lateral pleural spines.

These points may be taken up in order.

1. If trilobites have one appendage-bearing segment in front of the
mouth, they are Arachnida; if two, Crustacea. This is based on the
idea that in the course of evolution of the Arthropoda, the mouth has
shifted backward from a terminal position, and that as a pair of
appendages is passed, they lose their function as mouth-parts and
eventually become simple tactile organs. Thus arise the cheliceræ of
most arachnids, and the two pairs of tactile antennæ of most
Crustacea. This theory is excellent, and the rule holds well for
modern forms, but as shown by the varying length of the hypostoma in
different trilobites, the position of the mouth had not become fixed
in that group. In some trilobites, like _Triarthrus_, the gnathobases
of the second pair of appendages still function, but in all, so far as
known, the mouth was back of the points of attachment of at least two
pairs of appendages, and in some at least, back of the points of
attachment of four pairs. As pointed out in the case of _Calymene_ and
_Ceraurus_, the trilobites show a tendency toward the degeneration of
the first and second pairs of biramous appendages, particularly of the
gnathobases. They are in just that stage of the backward movement of
the mouth when the function of the antennæ as mandibles has not yet
been lost. If the presence of functional gnathobases back of the
mouth, rather than the points of attachment in front of the mouth, is
to be the guide, then Triarthrus might be classed as an arachnid and
_Calymene_ and _Isotelus_ as crustaceans. In other words, the rule
breaks down in this primitive group.

2. Superficially, the eyes of some trilobites do look like those of
_Limulus_, but how close the similarity really was it is impossible to
say. The schizochroal eyes were certainly very different, and Watase
and Exner both found the structure of the eye of the trilobite unlike
that of _Limulus_.

3. The importance of the trilobate form of the trilobite is very much
overestimated. It and the pygidium are due solely to functional
requirements. The axial lobe contained practically all the vital
organs and the side lobes were mechanical in origin and secondarily
protective. That the crustacean is not trilobate is frequently
asserted by zoologists, yet every text-book contains a picture of a
segment of a lobster with its axial and pleural lobes. It is a
fundamental structure among the Crustacea, obscured because most of
them are compressed rather than depressed.

4. The pygidium of trilobites is compared with the metasomatic shield
of _Limulus_. No homology, if homology is intended, could be more
erroneous. The metasomatic shield of _Limulus_ is, as shown by
ontogeny and phylogeny, formed by the fusion of segments formerly
free, and includes the segments between the cephalic and anal shields,
or what would be known as the thorax of a trilobite. No trilobite
has a metasomatic shield. The pygidium of a trilobite, as shown by
ontogeny, is built up by growth in front of the anal region, and since
the segments were never free, it can not strictly be said to be
composed of fused segments. Some Crustacea do form a pygidial shield,
as in certain orders of the Isopoda.

5. The post-anal spine of Dalmanites and some other trilobites is
similar to that of _Limulus_, but this seems a point of no especial
significance. That a similar spine has not been developed in the
Crustacea is probably due to the fact that they do not have the broad
depressed shape which makes it so difficult for a _Limulus_ to right
itself when once turned on its back. Relatively few trilobites have
it, and it is probably correlated with some special adaptation.

6. There is nothing among the trilobites comparable to the movable
lateral spines of the metasoma of _Limulus_.

While, as classifications are made up, the Trilobita must be placed in
the Crustacea rather than the Arachnida, there is no reason why both
the modern Crustacea and the Arachnida should not be derived from the
trilobites.

MEROSTOMATA.

It has been a custom of long standing to compare the trilobite with
_Limulus_. Packard (1872) gave great vitality to the theory of
the close affinity of the two when he described the so-called
trilobite-stage in the development of _Limulus polyphemus_. His
influence on Walcott's ideas (1881) is obvious. Lankester has gone
still further, and associated the Trilobita with the Merostomata in
the Arachnida.

The absence of antennules at any stage in development allies _Limulus_
so closely with the Arachnida and separates it so far from the
Trilobita that in recent years there has been a tendency to give up
the attempt to prove a relationship between the merostomes and
trilobites, especially since Clarke and Ruedemann, in their extensive
study of the Eurypterida, found nothing to indicate the crustacean
nature of that group. A new point of view is, however, presented by
the curious _Sidneyia inexpectans_ and _Emeraldella brocki_ described
by Walcott from the Middle Cambrian.

=Sidneyia inexpectans= Walcott.

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1911, p. 21,
     pl. 2, fig. 1 (not figs. 2, 3); pls. 3-5; pl. 6, fig. 3; pl. 7,
     fig. 1.

The body of this animal is elongate, somewhat eurypterid-like, but
with a broad telson supplied with lateral swimmerets. The cephalon is
short, with lateral compound eyes. The trunk consists of eleven
segments, the anterior nine of which are conspicuously wider than the
two behind them, and the telson consists of a single elongate plate.

On the ventral side of the head there is a large hypostoma and five,
pairs of appendages. The first pair are multisegmented antennules. The
second pair have not been adequately described. The third are large,
complex claws, and the fourth and fifth suggest broad, stocky
endopodites. Broad gnathobases are attached to the coxopodites of the
third to fifth pairs of appendages and form very strong jaws.

The first nine segments of the thorax have one pair each of broad
filiform branchial appendages, suggestive of the exopodites of
trilobites, but no endopodites have been seen. The tenth and eleventh
segments seem to lack appendages entirely.

=Emeraldella brocki= Walcott.

     Illustrated: _Sidneyia inexpectans_ Walcott _partim_, Smithson.
     Misc. Coll., vol. 57, 1911, pl. 2, figs. 2, 3 (not fig. 1);--Ibid.,
     1912, p. 206, text fig. 10.

     _Emeraldella brocki_ Walcott, Ibid., 1912, p. 203, pl. 30, fig. 2;
     text fig. 8;--Ibid., vol. 67, 1918, p. 118 (correction).

_Emeraldella_ has much the same shape as _Sidneyia_ and the same
number of segments, but instead of a broad flat telson, it has a long
_Limulus_-like spine. The cephalon is about as wide as long, and eyes
have not yet been seen. The body consists of eleven segments and a
telson (Walcott says twelve and a telson but shows only eleven in the
figures). Nine of the segments, as in _Sidneyia_, are broad, the next
two narrow.

The ventral side of the cephalon has a long hypostoma, and five pairs
of appendages. The first pair are very long multi segmented antennules
and the next four pairs seem to be rather slender, spiniferous,
jointed endopodites. Whether or not gnathobases were present is not
shown by the figures, but owing to the long hypostoma the appendages
are grouped about the mouth. All the segments of the body, unless it
were the telson, seem to have borne appendages. On the anterior end,
they were clearly biramous (1912, p. 206, text fig. 10), and that they
were present along the body is shown by figure 2, plate 30, 1912.

The present state of knowledge of both these peculiar animals leaves
much to be desired. The indications are that the cephalic appendages
are not biramous, and that only one pair of antennæ, the first, are
developed as tactile organs. The thoracic appendages of _Emeraldella_
are biramous, and also possibly those of _Sidneyia_. In the latter,
the last two abdominal segments seem to have been without appendages,
while in _Emeraldella_ at least one branch of each appendage, and
possibly both, is retained.

These animals, which may be looked upon as the last survivors of an
order of pre-Cambrian arthropods, have the appearance of an
eurypterid, but their dominant characteristics are crustacean. The
features which suggest the Eurypterida are: elongate, obovate,
non-trilobate, tapering body; telson-like posterior segment; marginal,
compound, sessile eyes; claw-like third cephalic appendages; and, more
particularly, the general resemblance of the test to that of an
eurypterid like _Strabops_. In form, _Sidneyia_ agrees with the
theoretical prototype of the Eurypterida reconstructed by Clarke and
Ruedemann (Mem. 14, N. Y. State Mus., vol. 1, 1912, p. 124) in its
short wide head with marginal eyes, and its undifferentiated body.
There is, moreover, no differentiation of the postcephalic appendages.

The crustacean characteristics are seen in the presence of five,
instead of six, pairs of appendages on the head, the first of which
are multisegmented antennules, and in the biramous appendages on the
body of _Emeraldella_. It should be noted that these latter are
typically trilobitic, each consisting of an endopodite with six
segments and a setiferous exopodite.

Clarke and Ruedemann (1912, p. 406) have discussed _Sidneyia_ briefly,
and conclude:

     It seems to us probable that the Limulava [_Sidneyia_ and
     _Amiella_] as described are not eurypterids but constitute a
     primitive order, though exhibiting some remarkable adaptive
     features. This order possibly belongs to the Merostomata, but is
     distinctly allied to the crustaceans in such important characters
     as the structure of the legs and telson, and is therefore much
     generalized.

The specialization of _Sidneyia_ consists in the remarkable
development of a highly complex claw on each of the third cephalic
appendages, and in the compound tail-fin, built up of the last segment
and one or more pairs of swimmerets. These two characteristics seem to
preclude the possibility of deriving the eurypterids from _Sidneyia_
itself, but it seems entirely within reason that they may have been
derived from another slightly less specialized member of the same
order.

That _Sidneyia_ is descended from any known trilobite seems highly
improbable, but that it was descended from the same ancestral stock as
the trilobites is, I believe, indicated by the presence of five pairs
of appendages on the cephalon and trilobitic legs on the abdomen.

=Molaria= and =Habelia.=

Other so-called Merostomata found by Walcott in the Middle Cambrian
are the genera _Molaria_ and _Habelia_, both referred to the Cambrian
family Aglaspidæ. These genera seem to conform with _Aglaspis_ of the
Upper Cambrian in having a trilobite-like cephalon without facial
sutures, a trilobite-like thorax of a small but variable (7-12) number
of segments, and a _Limulus_-like telson. Neither of them has yet been
fully described or figured, but (Walcott 1912 A, p. 202) _Habelia_
appears to have five pairs of cephalic appendages, the first two pairs
of which are multisegmented antennæ. The thoracic appendages are
likewise none too well known, but they appear to have been biramous.