THE OSTRACODA 799 



lated waters, although this fact has not received the attention it 

 should. Even though in general they seem to be no more sensitive 

 to their surroundirfgs than the Cladocera or the Copepoda, yet 

 there is no doubt that the amount of light, of pressure, of varia- 

 tions In temperature and composition of the water, the rate of 

 flow of the same, the nature of the bottom, and the presence or 

 absence of algae, etc., must certainly exert a real or intrinsic in- 

 fluence on the prosperous development of all these Entomostraca. 

 Direct or intense light certainly accelerates all their life processes, 

 as may be evidenced in the fact that all free and actively swim- 

 ming forms are quite likely to turn towards a source of light, or, 

 in other words, are positively heliotropic. Shady areas in pools 

 are not nearly so likely to contain the free swimming forms such 

 as Cypris, Cypria, Cyclocypris, and Notodromas except occasionally 

 or sporadically, while the lighter and sunnier areas of the same 

 body will contain them in abundance; in other words, the more 

 uniform the distribution of light, the more nearly uniform becomes 

 the distribution of any certain form. On the other hand, it seems 

 a general rule that the less able these forms are to swim, the greater 

 the certainty that they are confined to the deeper and darker 

 areas, in the ooze and slimy debris of the bottom. It also appears 

 that some species may be affected but little by depth, and there- 

 fore light and pressure; Cypridopsis vidua has been found in all 

 depths from i centimeter to fully 300 meters. 



Experience teaches that practically no forms are found in pure 

 spring water or in well water. Even so, we find practically none 

 in waters that have been polluted with dyes, or by chemical 

 means, although many different degrees of power of resistance in 

 this regard may be found. Some species may be enclosed in the 

 smaller glass aquaria and live almost indefinitely without change 

 of water, even though the water becomes quite foul. For example, 

 Cypria opthalmica has been known to survive in such aquaria long 

 after the larger forms have died. Cyclocypris laevis will also live 

 many months in water that has not been freshened. Some few 

 forms have been known to exist in sulphur waters, others in hot 

 springs and even in sewer drains. Cypris incongruens has been 

 found in a pond fed by the drainage from a barnyard manure 



