THE PHYLA ARTHROPODA AND ONYCHOPHORA 



Rg. 15.8. The compound eye of the 

 crayfish. A, longitudinal section of 

 the entire eve, showing relationships 

 of visual units (ommatidia), nerve 

 tracts and their ganglionic relays, and 

 the muscles bv which the eve is moved. 

 B, two adjacent ommatidia from an 

 eye in the light-adapted condition; in 

 dim light, the pigment in the "iris" 

 pigment ceils migrates distally and 

 that in the retinal pigment cells moves 

 toward the base. This removes the 

 light barriers between the ommatidia 

 and makes possible the formation of 

 superposition images. (Adapted from 

 H. Bernhards, 1916, ^eitschrifl fur 

 wissenschaftliche ^oologie, vol. 116.) 



^rr-'Y^Wm >-Cornea 



»¥^. ol .Cornea cells 



Crystalline 

 body 



"Iris" 



pigment 



cells 



Retinal 



pigment 



cells 



Reflector 



Crystalline 

 rod 



Retina 



Basal 

 membrane 



^ ~-~ Nerve fibers 



aquatic, and generally of a simpler and more primitive organization than 

 such forms as the crayfish. 



The Entomostraca. The four principal types of entomostracans are repre- 

 sented by the orders Branchiopoda, Ostracoda, Copepoda, and Cirripedia. 

 The majority of these are marine forms, although the Branchiopoda are 

 predominantly fresh-water animals. The Branchiopoda include the fairy 

 shrimps, such as species of the genus Branchinecta (Fig. 15.9). These animals 

 swim on their backs, propelled by coordinated beating of their numerous, 

 paddle-like thoracic appendages. Fairy shrimps occur typically in shallow, 

 temporary ponds of fresh water formed in early spring by melting snow and 

 may usually be found for only a brief period. The males of Branchinecta are 



437 



