92 Dwellers of the Sea and Shore 



tract ends. Two other appendages are found on the 

 left side, and a glance at their location and structure 

 leaves no doubt of their origin. They are vestiges of 

 what were once highly specialized locomotor append- 

 ages in the ancestral form of this animal. But now 

 these degenerated, feeble, bristle-fringed outgrowths 

 serve an entirely different purpose. In the case of the 

 male crab their chief use is for sweeping forward the 

 stercoraceous matter deposited in the shell; in that of 

 the female they serve the additional function of anchor- 

 ages for her extruded eggs. 



The only effective strength that the hermit crab is 

 capable of exerting to secure himself in his shell is con- 

 centrated in a small group of muscles extending along 

 the under side of the hind body, and it is for the pur- 

 pose of retaining this hold that this region has acquired 

 its remarkable formation. In short, his belly functions 

 as an organ of prehension; that is to say, a grasping 

 organ. 



It is apparent from the foregoing that the crab is 

 not only compelled for reasons of safety to adopt a 

 defensive covering, but when changing from one shell 

 to another he also must act with the utmost dispatch 

 if he is to avoid being seized by some predatory rover 

 who may be near. Still, although it may be with good 

 reason that the exchange is made quickly, it must be 

 said that ofttimes it appears utterly without reason 

 that the exchange is made at all. Now many motives, 

 real or fancied, may inspire him to leave one shell for 

 another, but there is only one distinct physical cause 

 that absolutely forces him to seek a new covering. This 

 cause arises from the simple fact that the crab grows. 



