104 THE BIOLOGY OF THE SEA-SHORE 



for instance, has lost its hard covering, and has become 

 spirally wound in accordance with the shape of the shell, 

 the abdomen thus being equivalent in function to the 

 columella muscle which previously attached the mollusc. 

 The swimmerets are very much reduced, except the sixth 

 pair, particularly the appendage of the left side, which has 

 the form of a hook, and is coiled round the columella. The 

 attachment of the crab to its shell is so secure that it cannot 

 be removed without damage. Some of the hermit crabs 

 (and other species of Crustacea as well) make a similar use 

 of plant structures. For instance, the deep water form 

 Pylodieles miersii found in the Indian Ocean inhabits hollow 

 pieces of bamboo. Since these are quite straight the animal 

 is perfectly symmetrical. The tropical Robber-crab {Birgtis 

 latro), the adult of which is practically entirely terrestrial, 

 frequenting the coasts of coral islands in the Indian and 

 Pacific Oceans, and living on coconuts, differs from the true 

 hermit crabs, to which it is closely related, in not making use 

 of a portable shelter, perhaps, as Caiman (191 1) suggests, 

 owing to the difficulty of obtaining one of suitable size. 

 The terga of the abdomen have become re-developed and 

 the body is strictly symmetrical. Notwithstanding this, a 

 vestige of the old habit remains, the crabs thrusting their 

 abdomens under logs or into holes among the roots of trees. 

 Issel (19 1 8) describes a habit similar to that of the hermit 

 crabs in the Isopod Zenohiana prismatica occurring on the 

 Ligurian Coast. This form, which is a relative of Idothea, 

 lives in tubes formed of plant debris, either fragments or 

 roots of Posidonia, or in straws and similar structures. Of 

 the seven pairs of thoracic limbs the fourth is rudimentary. 

 They are folded over the back, and at the extremity of the 

 limb are two patches covered with tubercles which aid the 

 Crustacean in sticking to its house. The habits, it is interest- 

 ing to note, show points in common with those of tubicolous 

 Annelids. For instance, the creatures are acutely sensitive to 

 changes in degree of illumination, the least shadow causing 

 them to withdraw into the tube. Apparently Zenobiana 

 excavates its tube by eating out the pith of the plant tissue. 



