140 PHYLUM C@LENTERA., 
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 (Jerisarc) of varying complexity. 
But the members of such a colony do not usually remain 
similar and equivalent. In Mydractinia, 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 
Fic. 66.—Colony ot Hydractenza on back of a Buccinum 
shell tenanted by a hermit-crab. 
are long, slender, mobile, sensitive, ften 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 Aydractinia 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 
