MUTUALISTS. 77 



selves on the narrow brancliial cavity of some decapod 

 crustacean, and as soon as they have entered, throw off 

 all their travelling baggage ; in fact, there is no other 

 means for them to gain admission ; their lot is identified 

 with that of their host ; they can no longer live without 

 him. The female only, it is true, thus renounces her 

 liberty ; she sacrifices herself, as usual, for her family, 

 while the male, far from giving himself up, preserves his 

 defensive arms, his claws, and his liberty. 



The crustaceans called Caprellas are perhaps not so 

 independent as they appear to be ; it is not impossible 

 that their place may be among the crustaceans now 

 under our consideration. They are often found, together 

 with the Tanais, on the bodies of cetaceans and chelo- 

 nians, on plagiostomous fishes, or in the midst of 

 colonies of Sertulariae. They also establish themselves 

 on buoys w^hen they are well covered with animal life ; 

 and we have discovered them in prodigious numbers on 

 a piece of cable which had lain at the bottom of the sea, 

 and the w^hole surface of which was covered with animals 

 of every kind. 



We may here mention the Pycnogonons, the Saphy- 

 rinaB, the Peltidiae, and the Hersiliae; these crustaceans 

 often crawl over the skins of their congeners,. but "without 

 ever renouncing their independence ; and they are all 

 more or less occupied with the toilet of their neighbours. 



We shall place in a second section some animals 

 which have been usually classed among parasites, 

 rather because of their living upon their neighbours 

 than on account of their mode of life. If it is necessary 

 in menageries to have keepers to cleanse the animals 

 themselves, it is as requisite to have others to keep the 



