NUTRITION IN THE ENTOZOA. 75 



which terminates in a globular sac. When this 

 sac, which may be regarded either as the stomach, 

 or as a general reservoir of nutriment, is fully dis- 

 tended with fluid, its sides are stretched, so as to be 

 reduced to a very thin transparent membrane, 

 having a perfectly spherical shape: after this globe 

 has become swollen to a very large size, the neck 

 yields to the distension, and disappears; and the 

 head can then be distinguished only as a small point 

 on the surface of the globular sac. It is impossible 

 to conceive a more simple organic structure than 

 this, which may, in fact, be considered as an isolated 

 living stomach. The Cootmrus, which is found in 

 the brain of sheep, has a structure a little more 

 complicated ; for instead of a single head, there are 

 a great number spread over the surface, opening 

 into the same general cavity, and, when the sac is 

 distended, appearing only as opaque spots on its 

 surface. 



The structure of the Sponge has been already 

 fully described ; and the course of the minute 

 channels pointed out, in which a kind of circulation 

 of sea water is carried on for the nourishment of the 

 animal. The mode by which nutriment is ex- 

 tracted from this circulating fluid, and made to 

 contribute to the growth of these plant-like struc- 

 tures, is entirely unknown. 



The apparatus for nutrition possessed by animals 

 belonging to the tribe of Medusce is of a peculiar 

 kind. I have already described the more ordinary 

 form of these singular animals, which resemble a 

 mushroom, from the hemispherical form of their 

 bodies, and their central foot-stalk, or pedicle. In 

 the greater number of species there exists, at the 



