WORMS. 



133 



Asteracanthion the lenses look like brilliant eggs, 

 " each in its own scarlet nest." 



In some species there are as many as two hundred 

 eyes; but there appears to 

 be no retina, so that they 

 can do little more than dis- 

 tinguish between light and 

 darkness. 



It is quite possible that in 

 some of the lower animals, 

 where the eye-spot is sup- 

 posed to consist merely of a 

 layer of pigment at the end 

 of a nerve, a lens may here- 

 after be discovered. 



In the Turbellaria* the 

 eyes, which were first noticed 

 by de Quatrefages, are numerous, and lie immediately 

 under the epithelium (skin). They consist of a certain 

 number of crystalline rods and corresponding retinal 

 cells, resting on a cup-shaped bed of pigment, and con- 

 nected with a nerve. There is often a group on each 

 side of the head, immediately over the brain. In 

 species which possess tentacles the eyes are generally 

 combined with them ; in others they are scattered over 

 the whole periphery of the body, and look in all direc- 

 tions. They differ greatly in size, and in the number 

 of rods and retinal cells — the larger tentacular eyes 

 having several; the small, scattered ones, which are 

 generally more deeply situated, even as few as two or 

 three. 



Fig. 87.— Eye of Asteracanthion (after 

 Haeckel). c, Cuticle ; e, epitlielium ; 

 I, lens ; p, pigment. 



* "Die Polycladen," Fauna und Flora des Golfes von Neapel, 1884. 

 Carriere, " Die Augen von Planaria," Arch, fur Mic. Anat, 1882. 



