CRAYFISHES AND THEIR RELATIVES 



161 



a cephalothorax and a clearly segmented abdomen. The lobster 

 also has two pairs of antennae, a pair of stalked eyes, a number of 

 pairs of mouth parts, a pair of big claws, four pairs of walking legs, 

 to the bases of which gills are attached, and a pair of swimmerets 



abdomen 



FIG. 113. The crab. 



on each of the segments of the abdomen except the last. In general, 

 lobsters are very much larger than crayfishes, one of the largest 

 known specimens weighing over twenty-three pounds. 



Less like the crayfish in appearance are the crabs, yet a care- 

 ful examination shows that these animals have practically all of 

 the characteristics mentioned hi the preceding paragraph. The 

 cephalothorax of crabs, however, is 

 usually wider than it is long (Fig. 

 1 13) , and the abdomen is much reduced 

 and is commonly folded in a groove be- 

 neath the cephalothorax. Few of the 

 crabs are able to swim ; usually they 

 crawl sideways by the help of their 

 four pairs of walking legs. 



"A curious modification of habit is shown in the hermit crab 

 (Fig. 114), which in early life backs into an empty snail shell which 

 aids in protecting it from its enemies. The abdomen, thus covered, 

 becomes soft and flabby. As growth proceeds the necessity arises 

 for a larger shell, and the crab goes ' house-hunting ' among the empty 

 shells along the shore, or it may forcibly extract the snail or other 

 hermit from the home which strikes its fancy." JORDAN and 

 HEATH, "Animal Forms." 



M 



FIG. 114. The hermit crab 

 in an empty snail shell. 



