The Appendages, Anatomy, and Relationships of Trilobites

by Percy E. Raymond

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.