HYDROZOA. ^t 



More frequently, however, it is composed, not, as 

 in Hydra, of a single polypite, but of several 

 similar structures, connected with one another by 

 means of a common trunk, or ' coenosarc' This 

 coenosarc may branch, presenting an erect tree-like 

 aspect, and in such cases is permanently attached 

 by means of the hydrorhiza which terminates its 

 proximal extremity (b, smdjig. 5, a). Often too, it 

 excretes from its outer layer a strong chitinous 

 investment, from which peculiar cup-shaped pro- 

 cesses, or ' hydrothecse,' serving as protective 

 envelopes for the delicate polj^ites, may be deve- 

 loped [fig. 4, c). In other members of the class 

 this firm layer has no existence, the coenosarc re- 

 maining soft, flexible, and highly contractile ; a 

 modification which prevails among the complex 

 oceanic Hydrozoa, creatures of great beauty and 

 delicacy of structure, whose graceful movements 

 through the element wherein they live are further 

 assisted by the ^ nectocalyces,' or swimming bells, 

 with which the hydrosoma may be provided 

 {d and e). In one group of these, the proximal 

 end of the coenosarc becomes transformed into a 

 peculiar organ termed the ' somatocyst.' In 

 others, this same extremity expands to form the 

 * pneumatophore,' or float, which enables its pos- 

 sessor to remain without effort near the surface of 

 the water (e). There are, also, simple oceanic 

 Hydrozoa, whose hydrosoma is represented by a 

 single nectocalyx, from the under surface of which 

 a pol}^ite is, as it were, suspended (/). Finally, 

 in another division of the group, the proximal 

 extremity of the polypite is modified so as to form 

 a special organ, the * umbrella,' which usually 

 simulates the function of a nectocalyx, though its 



