iS8 



Aquatic Organisms 



Fig. 94. An Ostracod {Cypris 

 virens), lateral and dorsal views, 

 (after Sharpe.) 



Some Ostracods are free- 

 swimming (species of Cypris, 

 etc.) and some (Notodromas ) 

 haunt the surface in sum- 

 mer; but most are creeping 

 forms that live among 

 water plants or that burrow 

 in the bottom ooze. In pools where such food as algae 

 and decaying plants abound Ostracods frequently 

 swarm, and appear as a multitude of moving specks 

 when we look down into the still water. 



Relict pools in a dry summer are likely to be found 

 full of them. Both sexes are constantly present in 

 most species of Ostracods, but a few species are repre- 

 sented by females only, and reproduce by means of 

 unfertilized eggs. 



The Cope pods are the perennial entomostraca of open 

 water. Summer and winter they are present. Three 

 of the commonest genera are shown in figure 95, toge- 

 ther with a nauplius — the larval form in which the 

 members of this group hatch from the egg. Nothing is 

 more familiar in laboratory aquaria than the little 



white Cyclops (fig. 96, swim- 

 ming with a jerky motion, 

 the female carrying two 

 large sacs of eggs. 



A more or less pear-shaped 



body tapering to a bifurcate 



tail at the rear, a single 



median eye and a pair of 



large swimming antennae at 



Fig. 95. Common copepods the front, and four pairs of 



c ^^^^',) C To^l thoracic swimming feet 



, ^iSZSrS&rSE beneath, characterize the 



dl & n2E the form ° f that appendage members of this group. 



