84 THE VITAL FUNCTIONS. 



extremity, provided with instruments for its 

 attachment to the surfaces it inhabits. 



The Hydatid (Fig. 248) is another parasitic 

 worm 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 Ccemirus, 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 Spoflge 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- 



