236 



Natural History of British Zoophytes. 



contractile at every point, so that they can change the figure of their 

 bodies from a globe to a cylinder, or distort it with strictures, and 

 can shorten and extend the tentacula at will, sometimes to an ex- 

 tent which is astonishing, although nothing like muscular tissue can 

 be detected in their structure.* When therefore they have occasion 

 to conceal themselves within their cells, they are not necessitated, 

 like the ascidian, to bend the body in order to obtain sufficient space 

 for the tentacula, but they shorten the body and the tentacula at the 

 same time, causing the one to assume a more globular form, and the 

 other to dwindle down to mere knobs or papillae (Fig. 3.)t The tenta- 

 cula, even when fully extended, have not the same appearance, — 



Fig. 3. 



they taper a little, and are roughened with minute warts generally 

 arranged in an imperfectly verticillate fashion, and in their evolution 

 they are less regularly campanulate, one or more being usually in 

 action and moving from the outline of the circle. The stomach is 

 not a distinct sac, but a simple cavity towards the centre of the bo- 

 dy, " neither figured nor limited by particular membranes," and 

 from which the indigestible remains of the food are ejected at the 

 same aperture by which it had entered, for the aperture in the base 

 of the stomach or intestine seems to be appropriated to other offices. 

 And in reference to its relation with the polypidom there is this dif- 

 ference, — the hydraform polype is not connected with the cell by 

 any membrane or ligament, but rather sits free within its miniature 

 cup, retained there only by the gelatinous living pedicle which is 

 prolonged from its base down the sheath, and binds all the polypes 

 of the polypidom in one sympathising family* 



But this description is applicable only to the Hydra itself, and to 

 those compound species which tenant the cups of the plant-like 

 polypidoms embraced in the order Zoophyta hydroida. The poly- 

 pes of the asteroid zoophytes, although evidently modelled on the 



* Trembley, Mem. pour l'hist. des Polypes, p. 25. Cams' Comp. Anat. i. p. 43. 

 •J- The figures represent Hydra viridis in various attitudes and states. 



