CRUSTACEA 



263 



Among the Entomostraca parthenogenetic reproduction is very 

 common. During the spring and summer, in our fresh-water ponds 

 and lakes, only the females of the common water fleas are to be found, 

 and generation after generation of female individuals come from thin- 

 shelled summer eggs which develop parthenogenetically in the brood 

 pouch of the mother. In the autumn males appear, and thick-shelled 

 winter eggs are produced, which are fertilized and live over the winter, 

 to hatch and produce females the following spring. Parthenogenetic 

 reproduction represents a rapid method of reproduction, which is neces- 

 sary if these animals are to maintain themselves, since they form a very 



Fig. 162. — Three types of entomostracans. A, a copepod, Cyclops sp. {From Bronn, 

 " Klassen und Ordnungen des Tierreichs," after Claus.) Female with eggs. B, an ostracod, 

 Cypris sp. (Modified from Thomson, "Outlines of Zoology") C. a cladoceran, Daphnia 

 pulex (de Geer), a very common and widely spread species. {Compiled from several figures.) 

 The figure represents a female with six eggs in the brood pouch. All figures highly magni- 

 fied. 



large element in the food of fishes and other animals. They are thus 

 indirectly of economic importance. 



The barnacles, which form an order of Entomostraca known as Cir- 

 ri pedia (sir i pe' di a; L., cirrus, curl, and pedis, foot), are Crustacea that 

 have become fixed and inclosed in a calcareous shell of several pieces. 

 The larvae are free-swimming but soon fasten themselves, back down, 

 upon a firm surface, grow shells, and remain for the rest of their lifetimes 

 attached (Fig. 163). If above the level of low tide barnacles close their 

 shells when the tide goes out but when the tide returns reopen them and 

 begin to collect food. This is brought to the mouth by the currents of 

 water created by the movements of the legs. They occur sometimes in 

 great numbers, completely covering the surfaces of rocks; younger 

 generations, graded in size, are attached to the surfaces of the older 

 generations so that 25 or 30 may exist within the area of only one square 



