/. TURBELLAR1A. 



269 



The alimentary canal (fig. 228) consists only of oesophagus 

 (pharynx) and mesenteron, the latter terminating blindly since no 

 intestine or anus is present. The mouth is on the lower surface, 

 at some distance from the anterior end, being occasionally in the 

 middle or even behind the middle of the body (fig. 231). It 

 leads into the muscular oesophagus, which is frequently enclosed 

 in a special sheath and then can be protruded like a proboscis. 





FIG. 228. 



FIG. 229. 



FlG. 228. Digestive and nervous systems of Syncoelidium pellucidum. (After 

 Wheeler.) a, alimentary tract ; 6, brain ; In, longitudinal (ventral) nerves ; m, 

 marginal nerve ; pi, longitudinal nerve of pharynx ; pi\ ring nerve of pharynx ; 

 <n, transverse nerve ; w, uterine ostium. 



FlG. 228.Pulychcerus caudatus. (After Mark.) 



The mesenteron, of entodermal origin, varies greatly in shape, its 

 modifications being made the basis of division of the class into 

 orders. In the Polycladidea there is a central portion from which 

 numerous branched caeca are given off; in the Tricladidea there 

 are three main trunks, each with lateral caecal diverticula; while in 

 the Ehabdocoelida the digestive tract is a simple rod-like sac, in 

 some cases ( Acoela) without internal cavity. The supra-oesophageal 

 ganglia always lie at the anterior end of the body, which is most 

 sensitive, and may be produced into feeler-like processes. This 



