98 MANUAL OF ANIMAL BIOLOGY 



The photoreceptor cells are characterized structurally by a network 

 of nerve fibrils in the cytoplasm and by the presence of a peculiar 

 transparent lens-like structure which serves to focus the light rays 

 on the nerve fibrils in certain regions of the cell. The sensory or- 

 gans of the head region are much more highly developed in the 

 marine worms, such as Nereis (p. 83), than they are in the Earth- 

 worms. 



For the reception of tactile, chemical, and thermal stimuli, there 

 are single sensory cells and also numerous and widely distributed 

 epidermal sense organs. Each one of these structures consists 

 of several sensory neurons which are very different from those of 

 the 'eye' just noted. They may be described as elongated cells 

 having a number of very tiny sense hairs at one end and a nerve 

 fiber at the other. They are so arranged that the sense hairs may 

 be projected to the exterior through definite openings, and thus 

 put in a favorable situation to receive the stimuli. 



TEXTBOOK BEFEBENCES 

 Woodruff, pp. 61-66 ; 78-83 ; 100-104 ; 263-267. 



Curtis and Guthrie, pp. 282-320, 140-152. 

 Guyer, pp. 206-214 ; 250-264. 

 Hegner, pp. 72-88 ; 199-222. 

 Newman, pp. 179-216; 270-276. 

 Shull, pp. 87 ; 89-102. 



GENEBAL BEFEBENCES 



Beddard. "Earthworms and Leeches," in the Cambridge Natural History 



(Macmillan) . 

 Benham. " Polychaet Worms," in the Cambridge Natural History (Macmillan). 

 Parker and H\swell. Textbook of Zoology (Macmillan). 

 Stephenson. The Oligochaeta (Oxford University Press). 



