Reed has recently (1916, pp. 122, 173) discussed these lines as developed in the Trinucleidae, 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 sh.e.l.l, 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 caeca, for they obviously were open tunnels ending blindly, whatever they contained.

The question of the function of the genal caeca 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 ca.n.a.l 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 ca.n.a.l behind the fifth pair of appendages. In the trilobites they debouch much further forward.

The princ.i.p.al 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 caeca 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 caeca 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 caeca 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 Harpedidae and Trinucleidae are h.o.m.ologous 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 caec.u.m 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 caeca. 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 caeca 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 caeca 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 caeca to the ocelli on the cheeks is best shown in the Trinucleidae. 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 h.o.m.ologue of the compound eye, and the eye-line here the h.o.m.ologue 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 princ.i.p.al 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 caeca are present in the adult, showing that they are different structures.

[Ill.u.s.tration: 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 caeca. These are the "facial sutures" of Ruedemann.]

_Harpides_ is another genus in which genal caeca 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 caeca 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 caeca 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 caeca 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 sh.e.l.l may not have been affected by them. While they indicate very fine, thread-like ca.n.a.ls, the present evidence seems to be in favor of a.s.signing to them the function of lodging the glands which secreted the princ.i.p.al digestive fluids.

HEART.

_Illaenus._

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 _Illaenus_ with the sh.e.l.l 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 _Illaenus_ is not stated, but since no _Illaenus_ 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 pa.s.sing allusion to this organ as an alimentary ca.n.a.l. This was the original opinion of Volborth. Pander, however, suggested to him that it might be a heart. The alimentary ca.n.a.l 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 ca.n.a.l of _Illaenus_ to be. Where the heart is 1.5 mm. to 3 mm. wide, the axial lobe is 11 mm. wide.

[Ill.u.s.tration: Fig. 26. Copy of Volborth's figure of the heart of _Illaenus_.]

[Ill.u.s.tration: Fig. 27. Heart of _Apus_. Copied from Gerstacker.]

While this may be merely a cast of the alimentary ca.n.a.l 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 ca.n.a.l. If the latter were filled with mud, and the animals lay on their backs, as most of them did at death, the ca.n.a.l 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, Agnostidae, Eodiscidae Trinucleidae, Harpedidae, Remopleuridae, Asaphidae Illaenidae, Goldiidae, Cheiruridae, and Phacopidae, 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 ca.n.a.l. 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 a.s.signed 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 a.s.sertion 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 Asaphidae, 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 sh.e.l.l 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 Asaphidae, 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 Trinucleidae. _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 zoaeal 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 zoaea 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 zoaea 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 zoaeal spine and the phylogenetic history of the Trinucleidae or Cheiruridae (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 ev.a.g.i.n.ates, 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 zoaeal 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 zoaeal 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.