DWELLINGS. 171 



flourish on its back become too long and impede or 

 delay its progress, it tears them off with its claws 

 and thoroughly cleans itself. The carapace being 

 quite clean, the animal finds itself too smooth and 

 too easy to distinguish from surrounding objects ; it 

 therefore takes up again fragments of algae and re- 

 places them where they do not delay to take root like 

 cuttings and to flourish anew. This culture is there- 

 fore intentional ; the crab directs it and arrests its 

 exuberance ; it is no more the victim of it than the 

 gardener is the slave of the vegetables which he 

 waters day by day. From generation to generation 

 this crab has acquired the habit, the instinct if one 

 prefers, of thus covering itself so that it may be con- 

 fused with neighbouring objects. Naturally it is 

 ignorant of botany, and knows nothing of cuttings. 

 If placed in an aquarium with little fragments of 

 paper it will seize them and place them on its back, 

 as it would have done with algae, without troubling 

 as to whether they become fixed or not. In spite of 

 this lack of judgment, we cannot fail to recognise 

 in this Ma'ia a certain ingenuity in self-conceal- 

 ment. 



The Sponge-crab {Dromia vulgaris) also practises 

 this method of shelter. It seizes a large sponge and 

 maintains it firmly over its carapace with the help of 

 the posterior pair of limbs. The sponge continues to 

 prosper and to spread over the Crustacean who has 

 adopted it. (Fig. 23.) The two beings do not 

 seem to be definitely fixed to each other ; the contact 

 of a sudden wave will separate them. When the 

 divorce is effected, the Dromia immediately throws 

 itself on its cherished covering and replaces it. M. 

 Kiinckel d'Herculais tells of one of these curious 



