82 HTDROZOA. 



live powers. Several species of Hydra have been 

 described under such names as H. viridis, H. 

 rubra, IT. vulgaris and H. fusca. These differ 

 in size, colour, the form of the body, or in the 

 relative proportions of the polypite and tentacles. 

 The polypite of H. vulgaris is cylindrical, its 

 colour variable, but usually orange-brown, and its 

 tentacles of moderate length. H. viridis has a 

 polypite of a grass-green tint, furnished with 

 comparatively short tentacula. H. fusca is larger 

 than either of these, its colour is deep brown, and 

 its tentacles very long and extensile ; the proximal 

 extremity of the polypite becoming suddenly 

 attenuated for about a third of its length. When 

 living Plydrse are removed from the water, they 

 appear to the eye as minute specks of jelly, which 

 quickly, however, recover their true form on re- 

 immersion. In confinement they readily thrive, 

 seeking the light and feeding voraciously. Spe- 

 cimens of the Hydra may be kept in glass vessels, 

 and their singular habits observed by the student, 

 with little difficulty. 



3. Wrder 2 : Coryniilje. — In Corymorjjha, 

 Vorticlava, and Myriothela, the hydrosoma, like 

 that of Hydra, presents only a single polypite, 

 but, in the greater number of Corynidw, it is 

 composite, exhibiting numerous polypites con- 

 nected by a coenosarc, which may be either erect 

 and branching, as in Cordylophora, or reduced 

 to a delicate creeping tube, as in Clava and Tri- 

 cliydra. A hydrosoma of this kind may be com- 

 pared to a Hydra in the act of budding, while as 

 yet the young zooids remain in connection with 

 the primitive polypite, by the hydrorhiza of which 



