The Appendages, Anatomy, and Relationships of Trilobites Part 12

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 antennae and one at the base of the maxillae. 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 Ca.n.a.l.

Beyrich (1846, p. 30) first called attention to the alimentary ca.n.a.l 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 s.p.a.ce 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 ca.n.a.l. 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 ca.n.a.l. 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 Inst.i.tute 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 ca.n.a.l two small vessels which pa.s.s over into the richly branched ma.s.s 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 a.s.sume 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 ca.n.a.l has also shared in this ventral inflection.

The posterior part of the ca.n.a.l 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 ca.n.a.l, 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 ca.n.a.l might then have consisted of a gizzard belonging to the oesophagus, and astomach proper or main digestive ca.n.a.l.

... 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 ca.n.a.l 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 h.o.m.ology 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 ca.n.a.ls is shown with remarkable clearness [with those of _Limulus_].

I have been able to convince myself that the existence of the two ca.n.a.ls 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 princ.i.p.al 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 princ.i.p.ally served to cover the hepatic appendages of the alimentary ca.n.a.l.

The cause of the incomplete development of the glabellar lobes lies, hence, in the intrusion of the alimentary ca.n.a.l, 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 Phacopidae 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 ca.n.a.l 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 s.p.a.ces in these two cla.s.ses of specimens do not, however, necessarily mean anything more than imperfect preservation.

[Ill.u.s.tration: Fig. 21.--Transverse slice through _Ceraurus pleurexanthemus_, to show the dorsal sheath above the abdominal cavity. Specimen 118. Traced from a photographic enlargement. 4.]

[Ill.u.s.tration: Fig. 22.--Transverse section through the cephalon of _Ceraurus pleurexanthemus_, showing the abdominal sheath and the large mud-filled alimentary ca.n.a.l (clear white). Traced from a photographic enlargement. Specimen 97. 3.3.]

[Ill.u.s.tration: Fig. 23.--Transverse section of the thorax of _Calymene senaria_, showing the large size of the mud-filled alimentary ca.n.a.l (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 ca.n.a.l 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 ca.n.a.l 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 ca.n.a.l is oval, nearly twice as wide as high, but it is obviously somewhat depressed. The original ca.n.a.l 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 ca.n.a.l has fallen in (dorsally) since death.

The sections show that in _Ceraurus pleurexanthemus_ the anterior part of the alimentary ca.n.a.l 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 ca.n.a.l must be connected with the mouth and a.n.u.s, some such restoration as that of Jaekel is indicated. No chitinous lining of the stomodaeum or proctodaeum 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 ca.n.a.l 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 ca.n.a.l 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 ca.n.a.l 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 ca.n.a.l 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 ca.n.a.l tapers posteriorly.

_Cryptolithus goldfussi_ (Barrande).

Ill.u.s.trated: Beyrich, Untersuch. uber Trilobiten, Berlin, 1846, pl.

4, fig. 1c.--Barrande, Syst. Sil. Boheme, 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 ca.n.a.l 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 ca.n.a.l 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 ca.n.a.l 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 proctodaeum can not be determined. The inside of the ca.n.a.l has somewhat of a l.u.s.tre 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 ca.n.a.l 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 proctodaeum can not yet be differentiated from the mesenteron, and only in _Cryptolithus_ has the posterior portion of the alimentary ca.n.a.l been seen. It is, there, merely a continuation of the mesenteron. The stomodaeum likewise has not been identified, but was probably a short gullet leading up from the mouth into the enlarged digestive cavity.

[Ill.u.s.tration: Fig. 24. Longitudinal section of _Ceraurus pleurexanthemus_, showing the probable outline of the alimentary ca.n.a.l 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 sh.e.l.l once established, the significance of different types of glabellae 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 glabellae of most ancient and all primitive trilobites. The free-swimming larvae 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 s.p.a.ce, 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 Conocoryphidae. 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 Olenidae, 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 a.s.sume that he considered the whole as due to branches of nerves.