798 FRESH-WATER BIOLOGY 



They molt many times before reaching maturity. The change 

 that takes place is most complete. The shell falls off, and all the 

 internal parts are shed, even to the minutest hairs. 



The nervous system is composed of a so-called brain or supra- 

 esophageal ganglion, and several other gangha and connecting 

 nerve structures. The most important branches lead to the eyes, 

 which are either double as in Notodromas (Fig. 1247), o^"; more 

 commonly, as a single median-dorsal pigment spot. 



The most common sense organs other than the eyes are found 

 on the second antennae (Fig. 1290 t). These resemble a club and 

 hence are often called ^'sense clubs." Other sense organs appear 

 on the second antennae of the male, especially such fornis as 

 Candona, Cypria, and Notodromas (Fig. 1298 c). 

 Most Ostracoda are omnivorous. Decaying 

 vegetation and small animals form a large part 

 of their diet. Cypridopsis has been observed 

 forming skeleton leaves. Some will eat their 

 own kind, if opportunity offers. While in cap- 

 tivity most forms will eat from thin slices of 

 potato. Notodromas is an exception to most 

 others, as it has the curious habit of swimming 

 back down and clings to the surface film in an 

 Fig. 1247. Notodromas mo- endeavor to obtain food. Some forms may also 



nacha (O. F. M.). (a) Lens; •' 



(di^ForebiLin^^^'''''^'''^' attack Hviug or dying animals. Instances are 

 also on record of their having attacked Melicerta 

 ringens, a common fixed rotifer. Thus they act principally as 

 scavengers, as their greediness and oftentimes great numbers 

 would constitute them no inefficient agent in the work of purifying 

 standing waters. 



The fresh-water Ostracoda entirely lack any such organ as a 

 heart. The respiratory process therefore takes place through the 

 entire upper surface of the body, and through the inner cell layers 

 of the shell. A number of respiratory plates are fastened to the 

 mouth parts, the motions of which keep up a continuous stream 

 of fresh oxygen-laden water pouring through between the valves. 



It is self evident that favorable or unfavorable life conditions 

 exert a striking influence on the distribution of Ostracoda in iso- 



