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 visceral cavity, 

 occupies the whole interior of the polyp. In some minute species 

 (Hydrse) it is a 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 lamella 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 lamellae 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 lamell* 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. 



