406 



ESSENTIALS OF ZOOLOGY 



stimulated and movement is indicated by the rate of stimulation of 

 successive ommatidia. The farther the object is from the eye, the 

 fewer ommatidia will be stimulated. The crayfish eye is often 

 tei-med a modified api^iendage because an antennalike structure will 

 regenerate in case an eye is mutilated. 



■* Cornea 



"' Corneagen cells 



- - • Crystalline cone ..--•- 



Distal retinal 

 pigment cells 



Proximal retinal 

 pigment cells 



------ Rhabdome ■ 



M 



a 



Basement 

 — membrane - 





^ -(- — -Nerve fibers '- 



Fig. 164. — Longitudinal section of ommatidia from eye of crayfish, a. Position 

 of pigment when iiglu is present : b, position ui pigmeni when in iIil- dark. i\u«.iee 

 tliat in tlie latter the dis al pigment is in the outward position and the proximal 

 pigment is concentrated inwardly. (From Hegner, College Zoology, The Macmillan 

 Company, after Bernhards. ) 



Metabolism 



The crayfish ingests principally flesh from bodies of fish, snails, 

 tadpoles, insects, and other animals, some caught alive and others 

 found dead. The maxillae and maxillipeds hold the morsels while 



