6o 



AXIMAL BIOLOGY 



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 lobsters are of 

 considerable importance because of 

 use as food. Small boys sometimes 

 catch crawfish, 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- 

 ing the first 

 year the craw- 

 fish sheds its 

 covering, or 

 molts three 

 times, and 

 once each year 

 thereafter. It 

 grows very fast 

 for a few days 

 just after molt- 

 ing, while the 

 FIG. 95. DEVELOPMENT OF A CRAB. covering is soft 



a, nauplius just after hatching; b,c,d, zoea; e, megalops; /, adult. an j extensible 



Question: Which stage is most like a crayfish? Compare 

 with metamorphoses of insects. SittCC it is at 



