EYES. 97 



studied the eyes of Isotclus gigas, Bathyurus longispinus, Calymenc/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 cir- 

 cular 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. 



"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. The lenses are closely packed, 

 minute, usually hexagonal in outline, flat on the outer and convex on the inner surface. Such eyes are best 

 developed in Asaphus, Illanus, Nileus, Bumastus, Proctus, etc." 



2. Eyes with biconvex lenses. 



The surface of the eye is a mass of contiguous lenses, covered by a thin membrane which is frequently 

 absent from the specimens, due to poor preservation. The lenses are biconvex, and being in contact with one 

 another, are usually hexagonal, although in some cases they nearly retain their globular shape. Such eyes 

 are found in Bury care, Peltura, Spharopthalmus, Ctenopyge, Goldius, Cheirurus, and probably others. 



II. Aggregate eyes. 



The individual lenses are comparatively large, distinct from one another, each lying in its own socket. 

 There is, however, a thin membrane, which covers all those in any one aggregate, and is a continuation of 

 the general integument of the body. This membrane is continued as a thickened infolding which forms the 

 sockets of the lenses. 



Such eyes are known in the Phacopidx only. 



III. Stemmata and ocelli. 



The stemmata are present only in Harpes, where there may be on the summit of the cheek two or three 

 ocelli lying near one another. Each, viewed from above, is nearly circular in outline, almost hemispheric, 

 glossy and shining. In section they prove to be convex above and flat or slightly concave beneath. The 

 test covers and separates them, as in the case of the aggregate eyes. 



The ocelli of the Trinucleidse and Eoharfes are smaller, and the detailed structure not yet investigated. 



Lindstroem concludes that so far as its facets or lenses are concerned, the eye of the trilobite shows the 

 greatest analogy with the Isopoda, and the least with Limulus. 



SUMMARY. 



The simplest eyes found among the Trilobita are the ocelli. These consist of a Simple 

 thickening of the test to form a convex surface capable of concentrating light. The simi- 

 larity in position of the paired ocelli of trilobites and the simple eyes of copepods has per- 

 haps a significance. 



The schizochroal eyes may well be compared with the aggregate eyes of the chilopods 

 and scorpions. The mere presence of a common external covering is not sufficient to prove 

 this a true compound eye, especially as the covering is merely a continuation of the general 

 test. 



