38 
STRUCTURE AND PHYSIOLOGY 
have not the same appearance, — they taper a little, and are 
roughened with minute warts generally arranged in an imper- 
fectly vertieillate fashion ; and in their evolution they are less re- 
gularly 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 body, 
66 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 poly- 
pidom there is this difference, — the hydraform polype is not 
connected with the cell by any membrane or ligament, but ra- 
ther 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 sympathizing 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 polypes of the Asteroid zoophytes, although evidently mo- 
delled on the same type, have made considerable advances to- 
wards complexity of organization, and their relation to the poly- 
pidom is entirely altered. Hitherto the polypidom has been, 
what its name imports, a cell for retreat in danger, and in ordi- 
nary an extravascular insensible sheath to protect the contained 
animal from the rude contact of the circumfluent element; but 
now we find it occupying an internal position, and instead of a 
covering it has become a sort of prop or skeleton to a fleshy 
crust in which the polypes are immersed. In the Alcyonium 
this interior support is scarcely to be recognized in some cal- 
careous spicula scattered through the central mass, but in Pen- 
natula it forms a bone stretched like a vertebral column from 
one extremity to the other, and in Gorgonia it is ramified into 
branches after the manner of a tree. It is this axis, under what- 
ever shape it appears, which is the true analogue of the polypi- 
dom of the ascidian and hydraform polypes, although the name 
certainly has no suitableness here, for the polypes not only cannot 
nestle in that which is uncellular, but they have no immediate 
