294 THE WONDER OF LIFE 



way — the one animal literally making a tool of the 

 other ! 



One of the most extraordinary cases of commensalism 

 is that described by Colonel Alcock as estabUshed between 

 an Indian Ocean hermit-crab, Paguristes typica, and a sea- 

 anemone of the genus Mamillifera. The sea-anemone 

 settles down on the hinder part of the young hermit-crab's 

 tail, and the two animals grow up together in a most inti- 

 mate manner, the spreading anemone forming ' a blanket 

 which the hermit-crab can either draw completely forward 

 over its head or throw haK-back as it pleases '. 



A very weU-known association is that between a hermit- 

 crab and a bright orange sponge, Suherites domuncula, 

 which spreads over the Gasteropod shell which the hermit- 

 crab has borrowed. The sponge is unpalatable to many 

 animals ; it is packed with strong needles of flint ; and 

 it has a pungent odour. For these reasons it must be of 

 advantage to its bearer, which it also very effectively 

 masks. It seems to dissolve away the Gasteropod shell, 

 but this is probably no disadvantage, since it lightens the 

 burden the hermit-crab has to carry. When the sponge 

 settles down on the shell inhabited by a hermit-crab which 

 has not reached its limit of growth, it wiU of course be 

 left behind when the Crustacean flits. It is quite possible 

 that the vacated shell with its associated sponge may be 

 picked up by a smaller hermit-crab in search of a new shel- 

 ter. The same sponge also grows on the back of Dromia 

 vulga/ris, a common crab, and some experiments made at 

 the Naples Zoological Station by Signor PoUmanti brought 

 out two very interesting facts, first, that the crab takes the 

 initiative ia getting the sponge on to its back, planting it 

 there itself, and second, that the sponge really affords its 



