326 FRESH-WATER BIOLOGY 



In rhabdocoels (Fig. 589) which include smaller forms, the mouth 

 may be placed at the anterior end or at various points on the 

 ventral surface. The pharynx is represented by three general 

 types, simple, bulbous, and plicate. In the bulbous type a muscu- 

 lar membrane divides the pharynx from the surrounding mesen- 

 chyma; the plicate form does not have the dividing membrane, 

 but consists of a cylindrical tube lying within a pharyngeal cavity 

 which opens to the exterior through the mouth. The simple and 

 plicate types of pharynx lie more or less lengthwise and the organ 

 appears as a tube parallel with the surface of the body. The 

 bulbous pharynx is more variable and includes three types, the 

 rosette-shaped, the cask-shaped (doliolif orm) , and the variable. 

 The intestine has the form of a simple sac; it consists of a blind 

 cylindrical tube, median in position. It is sometimes provided 

 with short lateral diverticula. The walls are thin. 



In triclads (Fig. 590) the mouth is on the ventral surface usu- 

 ally just posterior to the middle of the body. The pharyngeal 

 region ordinarily shows externally about the middle of the body, 

 either as a more heavily pigmented or as a lighter colored area. 

 The pharynx is a cylindrical, very muscular tube which lies within 

 the pharyngeal cavity except when protruded while feeding. 

 In a single genus, Phagocata, there are many pharyngeal tubes 

 instead of one. The intestine is thin- walled as in the rhabdo- 

 coels but has three main branches, a single one extending forward, 

 and two passing back, one on either side of the pharynx to the 

 posterior end of the body. Numerous lateral diverticula are 

 found especially in the anterior region. These may anastomose 

 with each other or remain distinct. 



The protonephridial system (water-vascular system or simple 

 kidney) possesses one, two, or four principal canals, with a general 

 antero-posterior direction. The number and position of the open- 

 ings is variable. The nervous system includes two principal brain 

 ganglia and two main longitudinal nerves with numerous lateral 

 branches. In many forms the longitudinal nerves may be seen as 

 two light lines on the ventral surface. 



Reproduction is both sexual and asexual. The Turbellaria 

 are hermaphroditic with the female organs distinct from the male. 



