/. TVRBELLABIA. 



269 



The alimentary canal (fig. 228) coiisists only of cesopliagns 

 (pharynx) and niesenteron, the latter terniinating 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 (iig. 231). It 

 leads into the muscular oesojihagus, which is frequently enclosed 

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



Fig. 229. 



Fig 228 —Digestive and nervous systems of SyncceUdiuni peUucidum. (After 

 Wlieeler.) a, alimentary tract ; b, brain ; In, longitudinal (ventral) nerves ; m, 

 marginal nerve ; pi, longitudinal nerve of pharynx ; pr, rmg nerve of pliarynx ; 

 tn, transverse nerve ; it, uterine ostium. 



Fig. ■M).—Pahjch(erus 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 cajca are given off; in the Tricladidea there 

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

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

 some cases ( Acojla) without internal cavity. The supra-resophageal 

 ganp-lia always lie at the anterior end of the body, which is most 

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



