While I have nothing to add to what has been written about the eyes of trilobites, this sketch of the anatomy would be incomplete without some reference to the little which has been done on the structure of these organs.
Quenstedt (1837, p. 339) appears to have been the first to compare the eyes of trilobites with those of other Crustacea. Johannes Müller had pointed out in 1829 (Meckel's Archiv) that two kinds of eyes were found in the latter group, compound eyes with a smooth cornea, and compound eyes with a facetted coat. Quenstedt cited Trilobites esmarkii Schlotheim (=Illænus crassicauda Dalman) as an example of the first group, and Calymene macrophthalma Brongniart (=Phacops latifrons Bronn) for the second. Misreading the somewhat careless style of Quenstedt, Barrande (1852, p. 133) reverses these, one of the few slips to be found in the voluminous writings of that remarkable savant.
Burmeister (1843; 1846, p. 19) considered the two kinds of eyes as essentially the same, and accounted for the conspicuous lenses of Phacops on the supposition that the cornea was thinner in that genus than in the trilobites with smooth eyes.
Barrande (1852, p. 135) recognized three types of eyes in trilobites, adding to Quenstedt's smooth and facetted compound eyes the groups of simple eyes found in Harpes. In his sections of 1852, pl. 3, figs. 15-25, which are evidently diagrammatic, he shows separated biconvex lenses in both types of compound eyes, Phacops and Dalmanites on one hand, and Asaphus, Goldius, Acidaspis, and Cyclopyge on the other. Clarke ( 1888), Exner ( 1891 ) and especially Lindstroem (1901) have since published much more accurate figures and descriptions. The first person to study the eye in thin section seems to have been Packard (1880), who published some very sketchy figures of specimens loaned him by Walcott. He studied the eyes of Isotelus gigas, Bathyurus longispinus, Calymene, and Phacops, and decided that the two types of eyes were fundamentally the same. He also compared them with the eyes of Limulus.
Clarke (1888), in a careful study of the eye of Phacops rana, found that the lenses were unequally biconvex, the curvature greater on the inner surface. The lens had a circular opening on the inner side, leading into a small pear-shaped cavity. The individual lenses were quite distinct from one another, and separated by a continuation of the test of the cheek.
Exner (1891, p. 34), in a comparison of the eyes of Phacops and Limulus, came to the opinion that they were very unlike, and that the former were really aggregates of simple eyes.
Lindstroem (1901, pp. 27-31) came to the conclusion that besides the blind trilobites there were trilobites with two kinds of compound eyes, trilobites with aggregate eyes, and trilobites with stemmata and ocelli. His views may be briefly summarized.
I. Compound eyes.
1. Eyes with prismatic, plano-convex lenses.