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 1'hist. des Polypes, p. 25. Cams' Comp. Anat. i. p. 43. 
f The figures represent Hydra viridis in various attitudes and states. 
