74 THE STUDY OF ANIMAL LIFE CHAP. 



Commensaljsm 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 partner- 

 ship known 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, some Ccelenterates, 

 and a few marine worms. 



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 clearly re- 

 cognised by Milne-Edwards in 1 827. 



Among the Stinging-animals there are many animal 



<_j cj ^j / 



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, psychic- 

 al] v united in the other ; but these differences are not 



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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 

 polyps, 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 polyps 

 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 result 

 from the fact that in such a large colony perfect uni- 

 formity of nutritive and other conditions is impossible. 

 Individuals which are fundamentally and originally like 



