Only a few authors have before now occupied themselves with the intimate structure of the trilobite eye. Packard gave in 1880, in the »American Naturalist» a note on the structure of the eye of trilobites (p. 503). There are some rough and inexact sketches of the eyes of Limulus and Asaphus, and although he seems to have known the beautiful researches of Grenacher he still »claims that the trilobite eye was organized on the same plan as Limulus». This statement is altogether wrong, and as I hope to show the trilobites have had eyes entirely different from that of Limulus and instead agreeing with those of the Isopoda and perhaps also with a few other Crustacea. In 1889 J. M. Clarke published an account[28] on the »Structure and development of the visual area in the trilobite Phacops rana Green». The aggregate eye described by him are of the type forming my third group. His holochroal division embraces my first and second groups and the schizochroal my third.
[28] Quart. Journ. of Morphology, vol. II p. 253.
The latest contribution to the knowledge of these eyes is found in Exner's »Physiologie der facettirten Augen von Krebsen and Insecten», 1891, where he gives good figures of the lenses of Phacops fecundus, pl. II figs 18, 19. He says that the palpable difference in the structure of these eyes and those of Limulus point to a change in the function of these eyes.
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. In the plurality it is, however, difficult to discern the lenses from the outside.
The lenses, as seen in a vertical section of the eye of Asaphus expansus, ([pl. I fig. 12]), are columnar prisms, like the pillars of basalt, attaining a length of 0,2 mm and at the point where the eye joins the test of 0,3 mm. At their interior extremity they have a breadth of 0,066 mm. On that point the surface is convex and at the exterior surface plane. They are closely packed and in a transverse section resemble a pavement of regular hexaeders. But they also assume other shapes and become rhombs or even quadrates, as seen in a specimen of Asaphus fallax ([pl. I fig. 18]), where the hexaeders and quadrates lie side by side without transitional forms. As a rule the lenses become more and more irregular in the vicinity of the surrounding frame or near the suture, nearly blotted out, as it were, and without any definite border line mingled with the confused, spongy mass that like a belt or a frame surrounds they eye in Asaphus and is sharply limited from the other part of the free cheek. This remarkable zone which is almost only present amongst the Asaphidæ (Asaphus, Megalaspis, Ptychopyge, Isotelus) retains in a confused manner somewhat of the prismatic structure of the eye as shown in the section ([Pl. I, fig. 11, b]). The eye of Bumastus also is environed by a similar zone, with a structure like that of the eye ([Pl. II fig 35, 41]).
In an undetermined species of Asaphus the lenses, although somewhat apart, are of an elongated hexaedric outline, which passes into a regular circular one farther away and on the surface of the eye they are slightly convex ([Pl. I figs 27-29]). In other genera belonging to this group the shape of the lenses are like those of Asaphus, so for instance in Illænus (I. chiron and I. Esmarki) and in Niobe. In Dysplanus centrotus they are shorter and broad, and their interior or lower surface strongly convex. It is likewise so in Nileus, where Nileus armadillo has an exceedingly thick exterior integument above the lenses. Such an integument has in a still higher degree increased in Bumastus sulcatus so as to exceed in thickness the stratum of the corneal prisms and it may in fact be doubted if the eyes of this species ever were able to function as visual organs. Proetus nearly resembles Bumastus in the thickness of the integument covering the prismatic lenses, which are interiorly convex, with a diameter of 0,03 mm.
In all genera belonging to this group a horizontal section gives the image of the hexaeders as in Asaphus with some change to squares or rhombs.
In scrutinizing a horizontal, somewhat extensive section of an eye in this group of trilobites, it will be perceived, as for instance in the figures ([Pl. V fig. 16, 22]) that the regular and evidently homogenous and intact prismatic lenses by and by have been altered and in a part of the section, a little distant from the intact ones disintegrated in their interior, showing various aspects of alteration. I cannot but think that this is a destruction which has set in long after the fossilization. It has revealed certain states of the intimate structure, certain delicate details, that now with an astonishing regularity come in sight and probably also lie hidden in the intact prisms. In the specimens of Asaphus, which we have studied, the alteration has taken the shape of a concentric stratification forming the body of the prisms, which is well discernible in a horizontal section, but not easy to detect in the longitudinal one ([Pl. I figs 9-10, 11]). It is likewise so in Niobe. In the other genera again the decomposition makes the prisms look like empty tubes in which a few irregular traverses and trabecular remains of their solid mass radiate towards the interior. They thus assume the aspect of a composite coral with its septa in the calicles ([Pl. VI fig. 31]). This is also evident in Nileus palpebrosus and Dysplanus.