FIELD BOOK OF PONDS AND STREAMS 



O straco ds — Ostracoda 



The ostracods have two-valved shells, hinged at the back 

 like clam shells, between which their slender tails kick rapidly 

 backward (Fig. 128). Their two shells are held together by 

 a transverse muscle as they are in clams, essentially the same 

 in its function as the muscles which we eat as fried scallops. 

 The ostracods constitute another army of minute crustaceans 

 averaging only a millimeter in length, and impossible to tell 

 apart with a simple lens. They are mostly creeping forms 



Fig. 128. — Common ostracod, Cypris, often found 

 in green alga mats: i, side, and 2, dorsal view. 



which frequent the sunny protected areas of ponds, and 

 intense light seems to quicken all their activities. They are 

 omnivorous scavengers, especially upon decayed vegetation; 

 few of them live in very clean water. They clamber about 

 on submerged plants, laying their eggs upon stems and on 

 the roots of duckweeds (Lemna) ; many live in the soft ooze 

 of the bottom. 



Subclass Malacostraca 



Scuds, Shrimps, Crayfishes 



The great majority of malacostracans live in salt water. 

 Many of them are the jointed shell fish well known, at least, 

 on the dinner plate — the prawns, shrimps, crabs, and lobsters. 

 Those of the fresh water, the sow-bugs, scuds, and crayfishes, 



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