PHYLUM CCELENTEKA. 



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 

 is common on shells at the shore, the colony consists of 

 polypes of varied structure and function. Some of the 

 polypes are nutritive "persons," like Hydra in appearance; 

 some are mouthless (?) reproductive " persons," which pro- 

 duce sperms and eggs, and so eventually start a new colony ; 

 others, with a mouth, are long, slender, sensitive, and 

 abundantly furnished with stinging cells ; while the little 



\^&&&mmm* 



,v 







FIG. 63. Colony of Hydractinia on back of a Buccinuni 

 shell tenanted by a hermit-crab. 



protecting spines at the base of the colony may perhaps be 

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

 necting canals at the base, and all are fed at the expense of 

 the nutritive " persons." Hydractinia thus exhibits poly- 

 morphism 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 polypes. These are in many 

 cases sessile and mouthless, or they may after a time 



