6o 



BEGINNERS' ZOOLOGY 



Fig. 93. — Crab from 

 below. 



Fig. 94. — Hermit Crab, 

 using shell of sea snail 

 for a house. 



Decapods. — All Crustacea which have ten feet belong 



in the order called decap'oda (ten-footed). This order 



includes the crabs, lobsters, shrimp, 

 etc. The crabs and the lobsters are of 

 considerable importance because of 

 use as food. Small boys sometimes 

 catch crayfish, and in some instances 

 are known to cook and eat them for 

 amusement, 



the only part cooked being the 



muscular tail. The crab's tail is 



small and flat and held under the 



body (Fig. 93). 



Since the limy covering to serve 



the purpose of protection is not 



soft enough to be alive and growing, it is evident that the 



Crustacea are hampered in their growth by their crusty 



covering. Dur- 



**^v f^slv"* m g the fi rst 

 year the cray- 

 fish sheds its 

 covering, or 

 moults three 

 times, and 

 once each year 

 thereafter. It 

 grows very fast 

 for a few days 



just after moul- 

 ting, while the 

 Fig. 95. — Development ok a Crab. covering is soft 



a, nauplius just after hatching; b,c,d, zoea; e, megalops; f, adult. iind extensible. 



Question: Which stage is most like a crayfish? Compare . . 



with metamorphoses of insects. OinCe It IS at 



