168 



INTRODUCTION TO ZOOLOGY 



remain are unequally developed on the two sides ; the last 

 swimmeret of the left side forms a kind of hook, with which 



the crab holds on to the central 

 column of the shell; the last 

 swimmeret on the right side is 

 smaller^ but used for the same 

 purpose. All the other appen- 

 dages of the right side of 

 the abdomen are absent or 

 very- rudimentary, but the 

 first three on the left side 

 of the body in the female 

 are fairly large and hairy, 

 being used by her for carry- 

 ing the eggs ; a fourth rudi- 

 mentary swimmeret may be 

 present further down the 

 abdomen. In the male, these 

 appendages are all very small. 

 It will be seen also that in 

 both sexes the fourth and 

 fifth "walking" legs are small 

 and do not project from the 

 shell. 



These hermit crabs can 

 withdraw almost entirely into 

 the shell, shutting themselves in by holding the much- 

 enlarged right claw over the shell entrance. 



Although so curiously asymmetrical when adult, these 

 hermit crabs begin life as little symmetrical zoom larvae, 

 very similar to those of ordinary crabs, though without the 

 long spines ; they remain symmetrical up to a stage corre- 

 sponding to the megalopa larva of crabs, when they have 

 five pairs of equally developed swimmerets ; they then give 

 up an active life and settle at the sea bottom, and the right 

 side of the body begins to degenerate and its internal organs 

 to shift in position. Now the larva finds a small shell to fit 

 it, and takes up its abode in it. As it grows too big for one 

 shell it searches for another, bigger, empty shell, or it may 

 actually turn out the rightful owner and forcibly take 

 possession of its shell. 



Fig. 104. — A I'emale Hermit Crab re- 

 moved from a Whelk Shell to show 

 the soft asymmetrical Ahdomen. 



