OF SIGHT. 



planaria and the nereis. In these latter animals there are 

 four spots. According to Miiller, they are small bodies, 

 rounded behind, and flattened in front, composed of a black, 

 cup-shaped membrane, containing a small white, opaque body, 

 which seems to be a continuation of the optic nerve. It cannot 

 be doubted, therefore, that these are eyes ; but as they lack 

 the optical apparatus which produces images, we must suppose 

 that they ran only receive a general impression of light, with- 

 out the power of discerning objects. 



140. Eye-spots very similar to those of the nereis are 

 found at the extremity of the rays of some of the star fishes ; 

 in the sea-urchins they are placed around the border of the 

 apical disc, and at the margin of many medusae, and in some 

 polyps. M. Ehreuberg has shown that similar spots also exist 

 in a large number of the infusoria. 



141. In all the animals mentioned above, the eyes, what- 

 ever their number, are apart from each other. But there is 

 still another type of simple eyes, known as aggregate eyes. 

 In some millipedes, the pill-bugs, for instance, the eyes are 

 collected into groups, like those of spiders ; each eye inclosing 

 a crystalline lens and a vitreous body, surrounded by a retina 

 and choroid. Such eyes consequently form a natural transi- 

 tion to the compound eyes of insects and Crustacea, to which 

 we now give our attention. 



142. Compound eyes have the same general form as simple 

 eyes ; they are placed either on the sides of the head, as in 

 insects, or supported on pedicles, as in crabs. If we examine 

 an eye of this kind by a magnifying lens, we find its surface com- 



Fig. 44. 



posed of an infinite number of 

 angular, usually six-sided facettes 

 (fig. 44) . If these facettes are re- 

 moved, we find beneath, a corre- 

 sponding number of cones (c), 

 side by side, five or six times as 

 long as they are broad, and ar- 

 ranged like rays around the op- 

 tic nerve, from which each one 

 receives a little filament, so as to 

 present, according to Miiller, 

 the following disposition. The 

 cones are perfectly transparent, 

 but separated from each other by walls of pigment, in sucl; 



