ii4 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



a protrusible proboscis, formed from the anterior 

 portion of the digestive tract; in some the mouth 

 does not lead into any distinct gastric space 

 (Convoluta), or there may or may not be a central 

 space (Mesostomum) : such forms, of course, obtain 

 their nutriment by intracellular digestion. The 

 tube, when distinctly formed, may be simple through- 

 out, and blind at the end opposite the mouth ; or 

 there may be a muscular pharynx, and the tube 

 may have a vent or anus. The tube may be 

 bifurcated in its hinder part (some Trematoda), or 

 may give off a large number of branches, which, as 

 in the fluke, ramify through the body, and either 

 end blindly, or communicate with one another ; in 

 the latter cases the gastric canals have also a circu- 

 latory function, just like the gastro vascular canals of 

 the Medusae. (See page 110.) 



The Nematohelmintlies have the mouth at one 

 end of their elongated body, and the anus not far 

 from the opposite end ; the digestive tube is perfectly 

 straight, and is strengthened anteriorly by a deposit 

 of chitin. The mouth, which sometimes (Gordius) 

 disappears during the course of development, but not, 

 curiously enough, until the worm has ceased to live an 

 endoparasitic life, is only provided with circumoral 

 bristles in such (Anguillulidae) as never pass any 

 part of their lives within other animals. Anteriorly 

 the tube is often widened out, well supplied with 

 muscles, and converted into a sucking apparatus. 



The Earthworms afford an example of how an 

 animal may atone for the absence of certain organs 

 by what may be really regarded as artificial means ; 

 though they live on all kinds of food, and especially 

 on leaves, they are without any organ by means of 

 which their food may be broken up ; to effect this 

 they swallow small stones, which, acted on by the 

 contraction of the muscles in the walls of that 



