38 STRUCTURE AND PHYSIOLOGY 



have not the same appearance, — they taper a little, and are 

 roughened with minute wart*=; generally arranged in an imper- 

 fectly verticiliate fashion ; and in their evolution they are less re- 

 gularly campanulate, one or more being usually in action and 

 movincr from the outline of the circle. The stomach is not a 

 distinct sac, but a simple cavity towards the centre of the body, 

 " 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, hut 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 snicula scattered throuoh 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 



