170 



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 Hydr actinia, 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 " per- 



FiG. 84. — Colony of Hydractinia on back of a Buccinum 

 shell tenanted by a hermit-crab. 



sons," with rudimentary tentacles, with or without a 

 mouth ; others are long, slender, mobile, sensitive, often 

 abundantly furnished with stinging cells ; while the little 

 protecting spines at the base of the colony may perhaps be 

 abortive " persons." All these polyps are united by con- 

 necting 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 Coelentera. 



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 



