CHAP. 17 Social Life of Animals 69 



anemone as a mask to the crab — and also perhaps as 

 aid in attack or defence — is obvious ; on the other hand, 

 the sea-anemone is carried about by the crab and may 

 derive food from the crumbs of its bearer's repast. 



CommensaUsm must be distinguished from parasitism, 

 in which the one organism feeds upon its host, though it is 

 quite possible that a commensal might degenerate into a 

 parasite. Quite distinct also is that intimate partnership 

 knowfn as symbiosis, illustrated by the union of algoid and 

 fungoid elements to form a lichen, or by the occurrence of 

 minute Algae as constant internal associates and helpful 

 partners of Radiolarians and some Coslenterates. 



2. Co-operation and Division of Labour. — The idea 

 of division of labour has been for a long time familiar to 

 men, but its biological importance was first satisfactorily 

 recognised by Milne-Edwards in 1827. 



Among the Stinging -animals there are many animal 

 colonies, aggregates of individuals, with a common life. 

 These begin from a single individual and are formed by 

 prolific budding, as a hive is formed by the prolific egg- 

 laying of a queen -bee. The mode of reproduction is 

 asexual in the one case, sexual in the other ; the resulting 

 individuals are physically united in the one case, psychically 

 united in the other ; but these differences are not so great 

 as they may at first sight appear. Many masses of coral 

 are animal colonies, but among the members or " persons," 

 as they are technically called, division of labour is very rare ; 

 moreover, in the growth of coral the younger individuals 

 often smother the older. In colonial zoophytes the 

 arborescent mode of growth usually obviates crushing ; and 

 there is sometimes very marked division of labour. Thus 

 in the colony of Hydractinia polypes, which is often found 

 growing on the shells tenanted by hermit-crabs, there may 

 be a hundred or more individuals all in organic connection. 

 The polypes are minute tubular animals, connected at 

 their bases, and stretching out from the surface of the shell 

 into the still water of the pool in which the hermit-crab is 

 resting. But among the hundred individuals there are 

 three or four castes, the differences between which probably 



