140 



PHYLUM CCELENTERA. 



imperfect separation of the units, continued indefinitely, we 

 can understand the formation of hydroid colonies, such as 

 the zoophytes. In such cases the colony is usually sup- 

 ported by an organic sheath (perisarc) of varying complexity. 

 But the members of such a colony do not usually remain 

 similar and equivalent. In Hydractinia, for example, which 

 often grows on a Gastropod shell tenanted by a hermit- 

 crab, the colony consists of polyps of varied structure and 

 function. Some of the polyps are nutritive "persons," 

 like Hydra in appearance ; some are reproductive " persons," 

 with rudimentary tentacles, with or without a mouth ; others 



FlG. 66. Colony ot Hydractinia on back of a Buccinum 

 shell tenanted by a hermit-crab. 



are long, slender, mobile, sensitive, often abundantly fur- 

 nished with stinging cells ; while he little protecting 

 spines at the base of the colony may perhaps be abortive 

 "persons." All these polyps are united by connecting 

 canals at the base. Thus Hydractinia exhibits polymorphism 

 among the members of the colony, and a tendency towards 

 more or less division of labour is common in the Ccelentera. 

 In most hydroid colonies the division of labour only 

 amounts to dimorphism \ there are reproductive " persons," 

 different from the ordinary polyps. These are in many 

 cases sessile and mouthless, or they may after a time 



