WORMS. 



135 



Fig. 89 represents an eye-dot of Nereis. ' In this 

 genus there are t^YO pairs of eyes, wliich differ some- 



rig, ss.— Anterior extremity of a freshwater worm {Boliemilla comata); after 

 Vejdovsky).* a. Eye; 6, brain; c, cuticle; hp, hypoderm; lb, tactile hair; 

 ne, nerve ; v, blood-vessel, 



what in structure, the lens in the anterior pair being 

 flatter, that in the posterior more conical. In Hesione 

 the difference is even more 

 marked.f In Polyophthalmus, 

 besides the eyes in the head, 

 there is, as already mentioned, 

 a series along the sides of 

 the body, which differ some- 

 what in structure from those 

 in the head. 



As a general rule, in the Annelids each eye contains 

 a single lens, but the cephalic eyes of Polyophthalmus, 

 according to Mayer, contain three. 



* "Sys. und Morph. der Oligochseten." 



t Graber, "Morph. Unt. iiber die Augeu der frei-lebenden Borsten- 

 wurmer," Arch. fUr Mic. Anal, 1880. 



Fig. 89.— Eye-dot of Nereis (after 

 MuUer). In B the pigment is 

 partly removed so as to show 

 the lens. 



