152 THE LIFE OF CRUSTACEA 



their possessors, although we can hardly do more than 

 guess at their special significance. Most Copepoda 

 have only a single eye in the middle of the head, 

 corresponding to the single eye of the nauplius larva, 

 and of far simpler structure than the paired com- 

 pound eyes of most other Crustacea. In many 

 plankton species, however, this simple eye becomes 

 much enlarged and complicated in various ways. 

 The three parts of which it is normally made up 

 may become separated from each other, and are 

 sometimes increased in number to five, while lenses 

 serving to concentrate the light are often developed 

 by thickening of the overlying cuticle. The most 

 elaborately constructed eyes are found in the family 

 Corycasidse. In Copilia (Fig. 53) a pair of eyes of 

 relatively enormous size are present. Each has in 

 front a large biconvex lens set at the end of a conical 

 tube which extends backwards to a smaller lens (like 

 a telescope with object-glass and eyepiece), behind 

 which, again, are the sensory cells, corresponding to 

 the retina, enclosed in a tube of dark pigment, the 

 whole apparatus being more than half the length 

 of the body. These eyes, although paired, do not 

 correspond to the paired compound eyes of other 

 Crustacea, but have arisen by the separation and 

 enlargement of two of the three divisions of the 

 typical median Copepod eye. 



A peculiarity of the paired compound eyes found 

 in plankton Crustacea of several different orders 



