COMMENSALISM AND PARASITISM. 151 



Hermit-crab, which has no sponge to protect it, 

 is provided for by the simple expedient of fre- 

 quent changes into a new home. In this case it 

 is arranged for by taking into the home, on what 

 we may call board wages, an efficient scavenger. 



In this remarkable association, then, no less 

 than four species belonging to four different 

 groups of animals are concerned. First of all 

 there is the Gastropod Mollusc, which forms a 

 shell for the Crustacean Hermit-crab to commence 

 life in, then there is the Sponge which protects, 

 and afterwards forms a shelter and home for the 

 Hermit-crab, and lastly, there is the Annelid 

 worm, which helps in its way to keep the house 

 clean in return for the scraps of food that fall 

 from the head partner's table. 



A very similar association has recently been 

 described by Bouvier from the Gulf of Aden and 

 Red Sea waters. A number of simple solitary 

 Corals were thrown into an aquarium by a French 

 naturalist, some falling on their sides and some 

 on their crowns, but he noticed that, after the 

 lapse of some time, they were all in the erect 

 position again with their crowns of tentacles 

 expanded in the water. On carefully watching 

 them he observed that at the base of each Coral 

 there was a little hole from which emerged a 

 small unsegmented Worm, belonging to a family 

 that usually exhibits sand-burrowing propensities. 

 These Worms were found to be the agents which 

 restored the Corals to their erect positions. The 

 advantage of this arrangement to the Worm was 

 two-fold : it brought it into direct contact with 

 the sand in which it searches for its food, and, 



