HYDROZOA. 27 



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 (6, and^^.^r. 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 poljrpites, may be deve- 

 loped {^fi{). 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 

 Hydrozoctf whose hydrosoma is represented by a 

 single nectocalyx, from the under surface of which 

 a polypite 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 



