56 



BRANCH ARTHROPODA. 



FIG. 8r. 



Eu \m gu' rus hern har' dug. 

 Hermit-crab. (Reduced.) 



drawing the animal backward through the water with 



great rapidity. The young Her- 

 mit-crab, though at first a perfect 

 zoea, soon looks about for an empty 

 mollusk's shell, of the proper size, 

 into which it backs. As growth 

 proceeds, the body becomes too 

 large for its "house." The ani- 

 mal now looks about and finds a 

 larger one ; if this be occupied 

 by a fellow Hermit, he attempts 



forcibly to eject him, and, if successful, crawls from his 



old habitation and establishes himself in his new. 



Some terrestrial, air-breathing Crabs are said to be 



sufficiently expert to climb 



trees. 



The Crabs proper have the 



abdomen small, and folded 



under the greatly expanded 



,, Zoea of Crab. (Magnified.) 



thorax. The young are at 



first active swimmers, passing as such the zoea stage.* 



FIG. 88. 



We now come to a few forms whose exact positions 

 are not known. They are not true Crustaceans, nor are 

 they insects ; but they seem to occupy an intermediate 

 position. With a single exception they are comparatively 

 of trifling importance in the animal economy, being gen- 

 erally small and of rare occurrence. Of the exception 

 we will now treat. 



* As growth proceeds in the soft parts, the hard shells of the crustaceans be- 

 come confining, when moulting is the only relief. This is effected by the shell 

 splitting open, generally along the back, when the animal crawls out and hides 

 its helpless self, until, in a few days, a larger armor has been formed. 



