12 ZOOPHYTES. 
cles, and distending the body for the reception of food. A few have 
powers of locomotion ; but they are commonly attached by their lower 
surface or extremity to the rocks or some other support, where they 
live on such chance-bits as are thrown in their way. 
4. The internal cavity, which we here style the wzsceral cavity, 
occupies the whole interior of the polyp. In some minute species 
(Hydre) it isa mere tubular sac, so simple in its nature, that the 
animal may be turned inside out, and still eat, digest, and perform all 
the functions of life as before. In other species it is divided verti- 
cally by thin fleshy lamelle growing from the sides, and the mouth 
opens first into a cylindrical organ, called the stomach, and thence 
into the general internal or visceral cavity. Within this cavity the 
water is received, by which the polyp distends by injection its body 
and tentacles; here also the animal fluids are aerated by air taken up 
from the imbibed water; and in the walls of the same cavity, or the 
fleshy lamelle when these exist, the germs or ovules are produced. 
In the lowest grade of these animals, the Hydra, we have then the 
simplest form of an internal cavity, so complex in many other animals ; 
and in the fleshy lamelle possessing germinal functions, that pro- 
ceed in the higher grades from its walls, we see represented the 
system of glands and the viscera generally, which have an analogous 
connexion, where present, with the walls of the internal cavity of the 
body. 
5. The mouth is a simple opening through the fleshy disk; and as 
there are no organs for trituration, the process of digestion consists in 
the unaided action of the gastric fluid, or what corresponds thereto in 
these animals. ‘The refuse is ejected through the mouth after diges- 
tion, this being the only opening to the internal cavity. What may 
be the separate functions of the stomach and visceral cavity in the 
process of digestion is not definitely known; but it is probable that 
the appropriation of the chyle to the nourishment of the polyp takes 
place through the latter, and the lacunal passages or openings com- 
municating with it. 
6. ‘The existence of nerves, or at least of something acting the part 
of nervous matter, is necessary, in order that these animals should 
possess the sense of touch; but examinations hitherto have detected 
no centre of nervous action and no distinct nervous cords.* The 
sensibilities of polyps are feeble, and their movements slow. 
* It has been stated that in the Actinia a nervous thread may be traced around the 
mouth, which sends fibrils into the tentacles. This requires farther confirmation, 
