ARTHROPODA 



8t 



tection: they choose the dried shell of some uni- 

 valve mollusc and live in it (Fig. 16). How far 

 the case is that they need a house because their 

 tails are soft, and how far the contrary is true 

 that their tails are soft because they live in a 

 house, it would be difficult to say. Readers of 

 another volume in this series, Professor Hickson's 

 "Story of Animal Life in the Sea," will remember 

 that the hermit-crab often offers a curious instance 

 of " commensalism " or partnership with other ani- 

 mals. The hermit- 

 crab was, in fact, 

 one of the earliest 

 instances in which 

 such a partnership 

 was observed, the 



FlG. 16. Hermit Crabs. A, Aniculus tyficus, from the Indo- 

 Pacific Seas, one-half of the natural size. B, Caternus ttdicen, 

 from the Indo-Pacific Seas, slightly enlarged. 



companion being in this case a sea-anemone 

 perched on the shell in which the crab lives. 



The true Crabs are called Brachyura, or 

 Short-tails; for obvious reasons, the tail of a 



