Structure And Habits Of Trilobites.
INTERNAL ORGANS AND MUSCLES.
Granting that the trilobite is a simple, generalized, ancient
crustacean, it appears justifiable to attribute to it such internal
organs as seem, from a study of comparative anatomy, to be primitive.
The alimentary canal would be expected to be straight and simple,
curving downward to the mouth, and should be composed of three
portions, stomodæum, mesenteron, and proctodæum, the first and last
with chitinous lining. In modern Crustacea, muscle-bands run from the
gut to part of the adjacent body wall, so that scars of attachment of
these muscles may be sought. At the anterior end of the stomodæum,
they are usually especially strong. From the mesenteron there might be
pouch-like or tubular outgrowths.
The heart would probably be long and tubular, with a pair of ostia for
each somite.
In modern Crustacea, the chief organs of renal excretion are two pairs
of glands in the head, one lying at the base of the antennæ and one at
the base of the maxillæ. Only one pair is functional at a time, but
these are supposed to be survivors of a series of segmentally arranged
organs, so that there might be a pair to each somite of a trilobite.
The nervous system might be expected to consist of a supracesophageal
"brain," comprising at least two pairs of ganglionic centers, and a
double ventral chain of ganglia with a ladder-like arrangement.
Besides these organs, a variety of glands of special function might be
predicted.
Reproductive organs probably should occur in pairs, and more than one
pair is to be expected. There is little to indicate the probable
location of the genital openings, but they may have been located all
along the body back of the cephalon.
It may be profitable to summarize present knowledge of such traces of
these organs as have been found in the fossils, if only to point out
what should be sought.
ALIMENTARY CANAL.
Beyrich (1846, p. 30) first called attention to the alimentary canal
of a trilobite, (_Cryptolithus goldfussi_,) and Barrande (1852, p.
229) confirmed his observations. A number of specimens of this species
have been found which show a straight cylindrical tube or its filling,
extending from the glabella back nearly to the posterior end of the
pygidium. It lies directly under the median line of the axial lobe,
and less than its own diameter beneath the dorsal test. At the
anterior end it apparently enlarges to occupy the greater part of the
space between the glabella and the hypostoma, but was said by the
early observers to extend only a little over halfway to the front.
Beyrich thought the position of the median tubercle indicated the
location of the anterior end.
Walcott (1881, p. 200) stated that in his experience in cutting
sections of trilobites it was a very rare occurrence to find traces of
the alimentary canal. The visceral cavity was usually filled with
crystalline calcite and all vestiges of organs obliterated. There
were, however, some slices which showed a dark spot under the axial
lobe, which probably represented the canal. In his restoration he
showed it as of practically uniform diameter throughout, and extending
but slightly in front of the mouth.
Jaekel (1901, p. 168, fig. 28) has produced a very different
restoration. His discussion of this point seems so good, and has been
so completely overlooked, that I will append a slightly abridged
version of a translation made some years ago for Professor Beecher.
The idea was, however, not original with Jaekel, as it was suggested
by Bernard (1894, p. 417), but not worked out in detail.
While considering the problem as to what organ could have lain
beneath the glabella of the trilobite, and while studying the
organization of living Crustacea for the purpose of comparison, I
found in the collections of the Geological Institute preparations
of _Limulus_ which seemed to me to directly solve the entire
question.
From the mouth, which lies at about the middle of the head shield,
the oesophagus bends forward, swells out at the frontal margin of
the animal at a sharp upward bend in order to take a straight
course backward after the formation of an enlarged stomach. Still
within the head shield there branch out from each' side of the
canal two small vessels which pass over into the richly branched
mass of liver lying under the broad lateral parts of the head
shield. After seeing this specimen, I no longer had the least doubt
that the head shield of the trilobites is to be interpreted in a
similar manner. The position of the hypostoma and gnathopods makes
it necessary to assume that the position of the mouth of the
trilobite lay pretty far back. If, therefore, this depends upon the
secondary ventral deflection of the oral region, as seems to be the
case, then it is a priori probable that the anterior part of the
canal has also shared in this ventral inflection.
The posterior part of the canal in the region of the segmented
thorax and pygidium is comparatively narrow, as shown long ago by
Beyrich; he represents only a thin tube which shows no swellings
whatever, and such are usually missing in Arthropoda.
As the glabella of most trilobites is regularly convex, there must
lie beneath it an organ running from front to back, which presses
the bases of the cephalic legs away from each other and down from
the dorsal test. An organ so extensive and unpaired, running thus
from front to back, can, among the Arthropoda, be regarded only as
an alimentary canal, for the swellings of the cephalic ganglia and
the heart are by far too small to produce such striking elevations
on the front and upper surface of the glabella. The canal might
then have consisted of a gizzard belonging to the oesophagus,
and astomach proper or main digestive canal.
... Among the trilobites there are two pairs of vessels on both
sides of the glabella which have precisely the same position with
reference to the supposed course of the alimentary canal as the
ducts of the hepatic lobes in _Limulus_. One observes in numerous
trilobites, although in different degrees of clearness and under
various modifications, a dendritic marking of the inner surface
of the cheeks which takes its rise at the lateral margins of the
glabella and spreads thence like a bush over the entire surface
of the cheeks. Exactly the same position is taken by the richly
branched hepatic lobes of _Limulus_ on the lower surface of the
head shield; a fact of special weight in favor of the homology
and similar significance of the two phenomena, is that in the
trilobites also, the anterior of the two main ducts is the larger,
the posterior the smaller. The striking similarity of the two
structures is shown by a comparison of the head shield of
_Eurycare_ [_Elyx_] from the Cambrian of Sweden, in which the
course of the canals is shown with remarkable clearness [with
those of _Limulus_].
I have been able to convince myself that the existence of the two
canals on each side is also the rule in other genera, even though
the posterior pair is frequently but feebly developed or completely
obscured by the anterior pair. In _Dionide formosa_, for example, I
find only the anterior pair, which is very large and divided into
two principal branches. From all these considerations it seems to
me no longer doubtful that the median elevation was caused by the
stomach and gizzard, and that the cheeks have principally served to
cover the hepatic appendages of the alimentary canal.
The cause of the incomplete development of the glabellar lobes
lies, hence, in the intrusion of the alimentary canal, and it makes
naturally the most effect where the gizzard spreads out and bends
into the stomach. This spot lies behind the frontal lobe, which is
hence increased in size according as the stomach increases in size;
in this way not only the foremost segments of the glabella become
enlarged, but also the following ones more or less pressed aside.
This process is easily followed phylogenetically and
ontogenetically.
From the latter point of view, the development of _Paradoxides_ is
very instructive. In a head shield 2.5 mm. long the whole anterior
part of the glabella is broadened, but the five pairs of lateral
impressions are clearly marked and the six segments of the head
bounded by them are all of about the same size. In a head shield
about 13 mm. long, the foremost segment is very much increased in
size, the jaw lobes pressed still further apart; in adult forms
both anterior segments are combined into the frontal swellings
of the glabella. In other groups this process proceeds
phylogenetically still further, so that among the Phacopidæ and in
_Trinucleus_, behind the frontal swelling of the glabella only the
last cephalic segment retains a certain independence. The frontal
lobe is thus no definite part, although it is as a rule composed of
the mesotergites of the first two cranidial segments.
This idea of an enlarged mesenteron certainly has much to commend it,
and such actual evidence as exists seems in favor of rather than
against it. The strongest, firmest, best-protected place in the whole
body of the trilobite is the cavity between the vaulted glabella and
the hypostoma. As Jaekel has said, it is far too large a cavity for
the brain, larger than would seem to be required for a heart, and what
else could be there but a stomach? As has already been pointed out,
Beyrich and Barrande found a pear-shaped enlargement of the alimentary
canal under the glabella of _Cryptolithus_. Longitudinal sections
through the glabella of _Calymene_ and _Ceraurus_ practically always
show the cavity there filled with clear crystalline calcite. One
actual specimen of _Ceraurus_ (Walcott 1881, pl. 4, fig. 1) shows the
cavity between the glabella and hypostoma entirely empty. The vacant
spaces in these two classes of specimens do not, however, necessarily
mean anything more than imperfect preservation.
[Illustration: Fig. 21.--Transverse slice through _Ceraurus
pleurexanthemus_, to show the dorsal sheath above the abdominal
cavity. Specimen 118. Traced from a photographic enlargement. × 4.]
[Illustration: Fig. 22.--Transverse section through the cephalon of
_Ceraurus pleurexanthemus_, showing the abdominal sheath and the large
mud-filled alimentary canal (clear white). Traced from a photographic
enlargement. Specimen 97. × 3.3.]
[Illustration: Fig. 23.--Transverse section of the thorax of _Calymene
senaria_, showing the large size of the mud-filled alimentary canal
(clear white). Traced from a photographic enlargement One appendifer
(also clear white) is shown. Specimen 153. × 3.3.]
_Ceraurus pleurexanthemus._
This species is taken up first, as it is the one shown in Walcott's
often-copied figure (1881, pl. 4, fig. 6). It is to be feared that too
many have looked at this figure without reading the accompanying
explanation, and have taken it for a copy of an actual specimen and
not a mere diagram, which it admittedly is. The evidence on which it
is based is comprised in eight transverse slices, one through the
glabella and seven through the thorax. Three of these have been
figured by Walcott: No. 27, 1881, pl. 3, fig. 7; No. 13, 1881, pl. 2,
fig. 3, 1918, pl. 26, fig. 14; No. 202, 1918, pl. 27, fig. 8. In all,
as can be seen by reference to the figures, the canal is partially
collapsed, and is much larger than is indicated in Walcott's
restoration. The other sections bear out the testimony of those
figured. One of these figured specimens (No. 27) and another figured
herewith (No. 118, see fig. 21) show an exceedingly interesting
structure which has previously escaped notice. The body cavity seems
to have had, in this region at least, a chitinous sheath on the dorsal
side. As shown especially in figure 21, this sheath impinges dorsally
and laterally against the axial lobe and thus furnishes a special
protection for the soft organs beneath, probably protecting them from
the strain of the dorsal muscles.
While there is no way in which the location of these sections in the
thorax can be positively determined, it is probable that they came
from the anterior end. In sections further back, supposed to be in the
posterior region of the mesenteron, no sheath is shown, but the canal
is nearly if not quite as large in relation to the size of the axial
lobe.
The single section through the glabella (specimen 97) is of course
important and fortunately well preserved (fig. 22). It shows the
dorsal sheath pressed against the inner surface of the axial lobe
along its middle portion, but diverging from it at the sides. The
section of the canal is oval, nearly twice as wide as high, but it is
obviously somewhat depressed. The original canal evidently filled
nearly the whole of the dorsal part of the glabella in this particular
region. Unfortunately, the connection with the mouth is not shown, and
the form of the hypostoma indicates that the section cut the glabella
diagonally, either in the anterior or posterior part, probably the
latter. In all these cases it should be remembered that the specimens
were found lying on their backs, and the canal has fallen in
(dorsally) since death.
The sections show that in _Ceraurus pleurexanthemus_ the anterior part
of the alimentary canal was large, filling the part of the glabella
below the heart; that the body cavity was provided with a chitinous
dorsal sheath extending back into the thorax; and that the posterior
portion of the mesenteron was likewise large and oval in section.
Since the alimentary canal must be connected with the mouth and anus,
some such restoration as that of Jaekel is indicated. No chitinous
lining of the stomodæum or proctodæum was found, but it is not certain
that any of the sections cut either of those regions.
_Calymene senaria._
Ten transverse sections and one longitudinal slice show the form of
the alimentary canal in _Calymene_. One of these has been figured by
Walcott (1881, pl. 1, fig. 9) but without showing the organ in
question.
The only section cutting the cephalon which shows any trace of the
canal is a longitudinal one (No. 141), which is not very satisfactory.
It has a large, nearly circular, opaque spot under the anterior part
of the glabella which may or may not represent a section across the
anterior end of the mesenteron. Three sections (No. 9, 115, 143) show
the dorsal sheath, the latter having the mud-filled canal beneath it.
The sheath arches across the axial lobe as in Ceraurus, leaving room
for the dorsal muscles at the sides and above it. In this region the
canal is large and oval in section. Six slices cut the mesenteron
behind the abdominal sheath (Nos. 39, 117, 148, 153, 62, 65) (see fig.
23). In the first four of these it is oval in section and large, but
not so large as in No. 143. In the last two, it is small and circular
in section, from which it is inferred that the canal tapers
posteriorly.
_Cryptolithus goldfussi_ (Barrande).
Illustrated: Beyrich, Untersuch. über Trilobiten, Berlin, 1846, pl.
4, fig. 1c.--Barrande, Syst. Sil. Bohême, vol. 1 1852, pl. 30,
figs. 38, 39.
Both Beyrich and Barrande have shown that from the posterior end of
the axial lobe to the neck-ring on the cephalon, the alimentary canal
in _Cryptolithus_ has a nearly uniform diameter of less than half the
width of the axial lobe. In front of the neck-ring, it enlarges, and
while its original describers state that it extends only about halfway
to the front of the glabella, Barrande's figure 39 shows it extending
quite to the front, and his figure 38 shows it fully two thirds of the
distance to the anterior end, as does Beyrich's figure of 1846.
The Museum of Comparative Zoology contains a single specimen of this
species from Wesela, Bohemia, which shows the course of the canal from
the middle of the pygidium to the anterior part of the glabella. The
enlargement appears to begin about halfway to the front of the
glabella and to be greatest at the anterior end. At the anterior end
of the glabella, the anterior end of the thorax, and the posterior end
of the pygidium, the canal is still packed full of a material somewhat
darker in appearance than the matrix, while the remainder of it is
open. A well defined constriction is present under the middle of the
next to the last thoracic segment, but whether this is accidental or
whether it indicates the point where the mesenteron discharges into
the proctodæum can not be determined. The inside of the canal has
somewhat of a lustre and there are three conical projections into it
on the median ventral line, a very small one in front of the neck
furrow, a larger one under the anterior part of the second segment,
and a third between the fourth and fifth segments.
_Summary._
The specimens of _Cryptolithus_ from Bohemia and of _Ceraurus_ and
_Calymene_ from New York seem to substantiate the claim of Bernard and
Jaekel that at the anterior end of the canal there was an enlarged
organ which occupied the greater part of the cavity of the glabella.
It appears that it extended into the thorax, and that above it and the
heart was a chitinous dorsal sheath. Behind the enlarged portion, the
mesenteron appears to have been of practically uniform diameter in
_Cryptolithus_, but to have tapered posteriorly in Ceraurus and
_Calymene_. The proctodæum can not yet be differentiated from the
mesenteron, and only in _Cryptolithus_ has the posterior portion of
the alimentary canal been seen. It is, there, merely a continuation of
the mesenteron. The stomodæum likewise has not been identified, but
was probably a short gullet leading up from the mouth into the
enlarged digestive cavity.
[Illustration: Fig. 24. Longitudinal section of _Ceraurus
pleurexanthemus_, showing the probable outline of the alimentary canal
and the heart above it. A restoration based on the slices described
above.]
The principle of the enlargement of the latter and its influence on
the dorsal shell once established, the significance of different types
of glabellæ becomes apparent. It will be remembered that the glabella
of the protaspis of most trilobites is narrow, and that the same is
true of the glabellæ of most ancient and all primitive trilobites. The
free-swimming larvæ and the free-swimming ancestors of the trilobites
were probably strictly carnivorous, lived on concentrated food, and
needed but a small digestive tract. As the animals "discovered the
ocean bottom" and began to be omnivorous or herbivorous, larger
stomachs were required, and so in the later and more specialized
trilobites the glabella became expanded latterally or dorsally, or
both, to meet the requirement for more space, until, in such Devonian
genera as _Phacops_, the cephalon was nearly all glabella.
GASTRIC GLANDS.
Jaekel's suggestion, quoted above, that the so-called "nervures" seen
on the under surfaces of the heads of some trilobites are really
glands for the secretion of digestive juices, is at least worthy of
consideration. Moberg, however (1902, p. 299), suggested that these
markings probably had something to do with the eyes rather than the
stomach. He says in part (translation):
In general we can now say that such features are common to all the
eyeless Conocoryphidæ. With the conocoryphs I include _Elyx_ and
consider Harpides as at least closely related. Similar impressions
are also found in forms with eyes, as, for instance, in the
Olenidæ, but here such radiate partly from the border of the eye,
partly from the front end of the glabella, partly from the [visual
surface of the] eye, and sometimes from the angle between the
occipital ring and the glabella. They therefore go out from such
different points that they can not possibly be branches of the
liver. It would also be very remarkable if such an important organ
should have been developed in a few eyeless forms, but have failed
to leave the least trace in the rest of the trilobites.
Lindstroem (1901, pp. 18, 19, 33; pl. 5. figs. 29, 31; pl. 6,
figs. 43-45) has discussed these markings and given beautiful
figures showing their appearance in _Olenus_, _Parabolina_, _Elyx_,
_Conocoryphe_, and _Solenopleura_. He decided that they were to be
explained as branches of the circulatory system, comparing them with
the veins and arteries of _Limulus_. He pointed out that there was a
coincidence between these markings and the position of the eyes, and
suggested a causal connection with the latter.
Beecher (1895 B, p. 309), also from a comparison with _Limulus_,
suggested that the eye-lines of _Cryptolithus_, _Harpes_,
_Conocoryphe_, _Olenus_, _Ptychoparia_, _Arethusina_, etc., probably
represented the optic nerves, and since the eye-lines are usually the
main trunks of the dendritic markings, it is fair to assume that he
considered the whole as due to branches of nerves.
Reed has recently (1916, pp. 122, 173) discussed these lines as
developed in the Trinucleidæ, and seems to accept Beecher's
explanation.
Three explanations of the "nervures" are thus current, and the authors
of all of them refer us to _Limulus_ as proving their claims! So far
as general appearance goes, the markings on the trilobites more
closely resemble the veins of a _Limulus_ than either the nerves or
"liver" of that animal. The veins, however, are not in contact with
the dorsal shell, but are buried in the liver and muscles, while the
arrangement of the arteries, which are dorsal in position, is quite
unlike what is seen in the trilobites.
The term nervures, as applied to these markings, is not only
misleading, but an incorrect use of one of Barrande's words, for by
nervures he meant delicate surface markings. Until the real function
of the organs which made these markings is definitely established, it
may be well to call them genal cæca, for they obviously were open
tunnels ending blindly, whatever they contained.
The question of the function of the genal cæca can not, in any case,
be settled by an appeal to _Limulus_, and it is doubtful if it can be
settled at all at the present time. Certain things tend to show that
Jacket's explanation is the most plausible, and these may be briefly
set forth.
Walcott (1912 A, pp. 176, 179, pls. 27, 28) has described specimens of
_Naraoia_ and _Burgessia_ in which similar markings are well shown,
and where they are obviously connected with the alimentary canal just
at the anterior end of the mesenteron. In _Burgessia_, which seems to
be a notostracan branchiopod, the trunk sinuses are very wide, and the
appearance is on the whole unlike that of any known trilobite. In
_Naraoia_, however, the markings are much finer and directly
comparable with those of _Elyx_. If my contention that _Naraoia_ is a
trilobite should be sustained, it might almost settle the question of
the "nervures." In _Burgessia_ these lateral trunks enter the main
canal behind the fifth pair of appendages. In the trilobites they
debouch much further forward.
The principal argument in favor of the interpretation of these
markings as nerves lies in their connection with the eyes. There is
considerable evidence to indicate that the eye-lines and the genal
cæca are two distinct structures, but because both originate from the
sides of the anterior lobe of the glabella, and both extend outward at
nearly right angles to the axis, or obliquely backward, they are, when
both present, coincident. Genal cæca occur on blind trilobites, on
trilobites with simple eyes, and on trilobites with compound eyes.
Eye-lines occur on trilobites with both simple and compound eyes, and
genal cæca may or may not be present in both cases. The morphology
of the ridge forming the eye-line in trilobites with compound eyes
is well known. It is abundantly proved by ontogeny that it is the
continuation of the palpebral lobe, and a development of the pleura of
the first dorsal segment of the cephalon. Lake, Swinnerton, and Reed
have tried to show that the eye-lines of the Harpedidæ and Trinucleidæ
are homologous with the eye-lines of the trilobites with compound
eyes, and that the ocelli on the cheeks are therefore degenerate
compound eyes.
The simplest form of the genal cæcum is seen in the blind _Elyx_
(Lindstroem 1901, pl. 6, fig. 43). The main trunk is at nearly right
angles to the axis, the increase in its width is gradual in
approaching the glabella, and an equal number of branches diverge from
both sides.
_Ptychoparia striata_ (Barrande 1852, pl. 14, figs. 1, 3) is an
excellent example of a trilobite with compound eyes and genal cæca. It
will be noted that the main trunk and the eye-line are coincident, and
that both on the free and fixed cheeks the branches are all on the
anterior side of the eye-line. Compare this with the condition in
_Conocoryphe_ (Barrande, pl. 14, fig. 8; Lindstroem, pl. 6, fig. 44),
and one sees there a main branch having the same direction as in
_Ptychoparia_ and likewise with all the branches on the anterior side.
At first sight this would seem to support the contention that these
lines do lead out to the eyes, since _Conocoryphe_ is blind, and the
main trunk leads practically to the margin. But although Conocoryphe
is blind, it has free cheeks, and the main trunk does not lead to the
point on those free cheeks where eyes are to be expected, but back
into the genal angles. And this direction holds in such diverse genera
(as to eyes and free cheeks) as _Harpes_, _Cryptolithus_, _Dionide_,
and _Endymionia_. In all these the genal cæca fade out in the genal
angles, and in none of them would compound eyes be expected in that
region. The coincidence of the eye-lines with the trunks of the
genal cæca in _Ptychoparia_ seems to be merely a coincidence. That
the markings which radiate from the eyes of _Ptychoparia_ and
_Solenopleura_ are not impressions made by nerves is obvious. That
they are of the same nature as the similar markings in the eyeless
trilobites is equally obvious. Ergo, they can not be nerves in either
case, and that they have anything to do with the eyes is highly
improbable. The eye was merely superimposed upon these structures.
The relation of the genal cæca to the ocelli on the cheeks is best
shown in the Trinucleidæ. In all species of _Tretaspis_ simple eyes
are present, and in most of them there are very narrow eye-lines. The
latter are occasionally continued beyond the ocular tubercle back to
the genal angle. A similar course is seen in _Harpes_. If the simple
eye is the homologue of the compound eye, and the eye-line here the
homologue of the eye-line in _Ptychoparia_, why does it continue
beyond the eye? In any case, it can not be interpreted as a nerve.
_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.