I. TURBELLARIA 



241 



part of the digestive tract (Fecampia) or by the development of anchoring 

 structures. Appearance, ciliation, etc., often give the smaller turbellaria 

 a resemblance to the ciliate Protozoa, so that the beginner is often con- 

 fused between them. 



The alimentary canal (figs. 59, 215) consists only of oesophagus 

 (pharynx) and mesenteron, the latter terminating blindly since no 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. It leads into the muscular oesophagus, which fre- 

 quently can be protruded like a proboscis. The mesenteron, of ento- 

 dermal 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 (fig. 215) there are three main trunks, each with lateral ca^cal 

 diverticula; while in the Rhabdocoelida the digestive tract is a simple 

 rod-like sac, in some cases (Acoela) without internal cavity. The supra- 

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



- 



ffillfi 





w 



' ' 



- 



- 



- 

 



, 



^~*~^''-^ . " 



FIG. 216. Larva of Stylochus pilidium (from Korschelt-Heider, after (lotto). /', 

 enteron; En, remains of entoderm cells; S, resophagus. 



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

 region usually bears two or more simple eyes, and in a few a single 

 statocyst. Most of the turbellaria have numerous rhabditcs and rJiani- 

 ni/es, rod-like structures of varying shape and structure, in the skin. 

 The nettle cells found in the skin of many species (Microstomidae) are 

 derived from the hydroids eaten. 



The hermaphroditic sexual organs (fig. 75) and the excretory system 

 vary considerably. The eggs are usually large and are fastened by a stalk 



16 



