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.
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.
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.