

THE ARTHROPODS 149 



of swift swimming renders them delightful occupants of the 

 aquarium, where, however, they demand some care. Note 

 that the great claws are here carried stretched out as in a 

 lobster, not in the bent position as in the porcelain crab and the 

 true crabs. As in the porcelain crab, however, the last walking 

 leg is rudimentary, and the body is much broader and flatter 

 than in the lobster. In detail you will notice a number of curious 

 resemblances to porcelain crabs and differences from a lobster. 



More distantly related to the porcelain crabs, and more 

 familiar if not commoner forms are the hermit crabs, of which 

 one species (Pagurus bernhardus) is very common in shore pools. 

 The most interesting point in regard to it is that the abdomen or 

 tail is not here tucked up out of harm's way as in the porcelain 

 crabs, but trails lank and soft behind the hard anterior part of 

 the body. As it is this soft tail which contains many of the 

 organs of the body, and as it is useless for swimming and only 

 a danger if exposed, the hermit seeks some means of protection. 

 This is usually found in an empty gasteropod shell. Examine 

 an empty shell of periwinkle or whelk, and you will note that 

 the shell is coiled round a central pillar called the columella. 

 The hermit's tail is twisted so that it fits readily into the shell, 

 and it ends in a strong hook which can be fixed to the columella 

 of the shell, and so attaches the hermit to its borrowed house. 

 The chief disadvantage is that the hermit grows and its borrowed 

 shell does not, so that periodic house-moving is necessary, and 

 also that certain fish with a keen appreciation of hermits will 

 swallow shell and all in order to obtain the desired morsel. Her- 

 mits oppose various devices to this last danger. Some decorate 

 their shell with a sea-anemone having powerful stinging cells. 

 The anemone would sting the lips of the fish, and thus no doubt 

 protects the hermit. The difficulty is that when the hermit 

 moves it must take its anemone with it. Others decorate the 

 shell with sponge, which being full of prickles is again distaste- 

 ful to fish, but in this case it is difficult to move the messmate 

 to a new shell, and the hermit has to abandon its shell as this 

 grows too small, and content itself with living in a hollow of the 

 sponge. Other hermits live in shells covered with zoophytes (see 

 Fig. 83, p. 1 66), which fish again dislike, but here also moving 



