NUTRITION IN MEDUSiE. 65 



The Hydatid^ (I'^^g- 248,) is another parasitic worm, of 

 the simplest possible construction. It has a head (o,) of 

 which H is a magnified representation, furnished with four 

 suckers, and a tubular neck, which terminates in a globular 

 sac. When this sac, which is the stomach, is fully distend- 

 ed with fluid, its sides are stretched, so as to be reduced to 

 a very thin transparent membrane, having a perfectly sphe- 

 rical shape; after this globe has become swollen to a very 

 large size, the neck yields to the distention, 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 con- 

 ceive a more simple organic structure than this, which may, 

 in fact, be considered as an isolated living stomach. The 

 Cceniu'us, which is found in the brain of sheep, has a struc- 

 ture a little more complicated; for, instead of a single head, 

 there is a great number spread over the surface, opening into 

 the same general cavity, and when the sac is distended, ap- 

 pearing only as opaque spots on its surface. 



The structure of the Sponge has been ah'eady fully de- 

 scribed; 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 nutri- 

 ment is extracted from this circulating fluid, and made to 

 contribute to the growth of these plant-like structures, is 

 entirely unknown. 



The apparatus for nutrition possessed b}' animals belong- 

 ing to the tribe of Medusse is of a peculiar kind. I have 

 already described the more ordinary form of tliese singular 

 animals, which resembles a mushroom, from the hemispheri- 

 cal form of their bodies, and their central foot-stalk, or pedi- 

 cle. In the greater number of species there exists at the 

 extremity of this pedicle, a single aperture, which is tiie be- 

 ginning of a tube leading into a large central cavity in the 

 interior of the body, and which may, therefore, be regarded 

 as the mouth of the animal: but in those species which have 

 no pedicle, as the Equorca, the mouth is situated at the 

 centre of the under surface. The aperture is of sufiicienl 



Vol. II. 9 



