OF SIGHT. 53 



them, and even the choroid, which presents itself in the 

 form of a black ring around the crystalline lens. Many in 

 sects, in their caterpillar state, also have simple eyes. 

 1 85. Rudiments of eyes have been observed in very 

 many of the worms. They generally appear as small 

 black spots on the head ; such as are seen on the head 

 of the Leech, the Planaria and the Nereis. In these latter 

 animals there are four spots. According to Muller, 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 can 

 only receive a general impression of light, without the power 

 of discerning objects. 



86. 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, at the mar- 

 gin of many Medusae, and in 

 some Polypi. Ehrenberg has 

 shown that similar spots also 

 exist in a large number of the 



Infusoria. Fig. 17. 



87. In all the above-mentioned animals, 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 of the 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 



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