370 



APPLIED BIOLOGY 



rapidly as has the head-thorax part of the body (see Fig. 120), 

 and hence the greater part of an adult crab's body is head- 

 thorax. Because the abdomen is short, the crabs are often 

 called the " short-tailed decapods," while the lobster and 

 others are called " long-tailed." Such facts of structure lead 

 us to think that the various species of crabs are closely related 



FIG. 119. Larva of FIG. 120. Young crab after metamorphosis is 



crab, at, antennee ; complete. Note short abdomen in comparison 



a, abdomen; c, cara- with that of the larva in Fig. 119. (From 



pace. Brooks.) 



to each other, and more distantly related to the long-tailed 

 decapods. 



(D) A small permanent collection (in alcohol) of specimens of 

 various species of crayfishes, prawns, shrimps, and crabs would be 

 valuable for illustrating the general facts of structure which seem to 

 indicate relationship between these higher crustaceans. 



314. Hermit-crab. One of the most interesting cases 

 of degeneration among crustaceans is that of the hermit- 

 crabs, one species of which is shown in Figs. 121 and 122. 

 A young hermit-crab takes possession of a small shell (formed 

 by a snail which has died), and backing into it, twists the 

 abdomen so as to hold itself firmly in the shell, which it drags 

 about as it moves from place to place. When growth of the 



