Water- Fleas 



185 



Water-fleas — The most common of the Branchiopods 

 are the water-fleas (order Cladocera) such as are shown 

 in outline in figure 91. These are smaller, more trans- 

 parent forms, having the body, but not the head, in- 

 closed in a bivalve shell. The shell is thin, and finelv 

 reticulate or striated or sculptured, and often armed 

 with conspicuous spines. The post-abdomen is thin and 

 flat, armed with stout claws at its tip and fringed with 

 teeth on its rear margin, and it is moved in and out 

 between the valves of the shell like a knife blade in its 

 handle. The pulsating heart, the circulating blood, the 

 contracting muscles, and the vibrating gill-feet all show 

 through the shell most 

 clearly under a microscope. 

 Hence these forms are very 

 interesting for laboratory 

 study, requiring no prepara- 

 tion other than mounting 

 on a slide. 



Some water-fleas, like 

 Simocephalus, shown in fig- 

 ures 91 and 92 swim freely 

 on their backs, in which 

 position gravity may aid 

 them in getting food into their mouths. When the 

 swimming antennae are developed to great size, as in 

 Daphne (fig. 91a), the strokes are slow and progress is 

 made through the water in a series of jumps. ' When 

 the antennae are shorter, as in Chydorus (fig. gib), their 

 strokes are more rapidly repeated, and progression 

 steadier. 



The Cladocerans are abundant plancton organisms 



throughout the summer season. They forage at a little 



depth by day, and rise nearer to the surface by night. 



The food of water-fleas is mainly the lesser green 



algae and diatoms. They are among the most important 



Fig. 91. Water-fleas 



a. Daphne; b, Chydorus; c, Simocephalus; 

 d, Bosmina. Note the "proboscis." 



