CRUSTACEANS 



are but few compared with them. These are in general the 

 larger fresh water crustaceans, and though some of them 

 measure but a fraction of an inch, others are more than four 

 inches long. Different species are very differently shaped, 

 hardly recognizable as nearly related animals, yet the ground- 

 plan of their bodies is strikingly similar. In Asellus (Fig. 131) 

 as well as in a crayfish there are twenty body segments, joined 

 in various ways along the back, each one bearing a pair of 

 appendages. Five of these pairs are borne on the head, eight 

 on the thorax, and seven on the abdomen and they function 

 as feelers, eating tools, walking-legs, swimming paddles, and 

 reproductive organs. The male crayfish transfers sperm 

 cells to the female with the first pair of appendages on the 

 abdomen; and the female carries her eggs on her swimmerets. 

 Unlike as these appendages first appear, they prove to be 

 strikingly similar when they are carefully compared. 



Habitat. — Different orders of malacostracans live in differ- 

 ent places and will be discussed separately. Most of them 

 prefer small streams and quiet waters, but there are marked 

 exceptions; they all hide away from the light, and are often 

 quite unknown in places where they are abundant. Many of 

 them take to underground life and all but one of the orders 

 include species which live in caves, several being blind. 



Food. — They are omnivorous, but prefer dead and decaying 

 matter, and hence are very useful scavengers. 



Enemies. — Amphipods are eaten by fishes^ birds, and aqua- 

 tic insects. The "Caledonia shrimp," Gammarus limnceus, is 

 a favorite food of brook trout. 



Scuds — Amphipoda 



Amphipods are accomplished water acrobats and can climb, 

 jump, swim, or glide with equal ease. They are shaped like 

 fleas with arched backs and narrow bodies, with climbing 

 legs on the thorax, and swimming and jumping appendages 

 on the abdomen (Fig. 129). 



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