EYES. 97 
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 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, IllcEnus, Nileus, Bumastus, Pro'ctus, 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 Eurycare, 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 Phacopidae only. 
III. Stemmata and ocelli. 
The stemmata are present only in Harpcs, 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 Trinucleid^e and Eoharpcs 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. 
