84 THE VITAL FUNCTIONS. 



extremity, provided with instruments for its at- 

 tachment to the surfaces it inhabits. 



The Hydatid (Fig. 248) is another parasitic 

 vrorm of the simplest possible construction. It 

 has a head (o), (of which h is a magnified repre- 

 sentation,) furnished with four suckers, and a 

 tubular neck, which terminates in a globular 

 sac. When this sac, which is the stomach, is 

 fully distended 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 distin- 

 guished 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 Ccenurus, which is found in the 

 brain of sheep, has a structure a little more com- 

 plicated ; 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 circu- 

 lation of sea water is carried on for the nourish- 

 ment of the animal. The mode by which nutri- 



