142 



COMPARATIVE ANATOMY 



CHAP. 



especially in long forms, in very numerous pairs. The unpaired 

 canal runs forward from the front end of the stomach intestine 

 in the middle line of the body. 



In a few Polydada the gastro-canals open on the dorsal surface of 

 the body (Yungia) through pores, or else on the edge of the body 

 (Cydoporus). In Thysanozoon diverticula of the intestinal branches 

 run into the lobate processes of the dorsal surface. 



B. Trielada (Fig. 104). A distinction between main intestine and 

 intestinal branches cannot here be made. Three gastro-canals start 



FIG. 105. Intestinal 

 and nervous systems of 

 Mesostoma (Rhabdo- 

 ccele). ph, Pharynx; d, 



intestine. 



FIG. 104. Intestinal and nervous 

 systems of a fresh-water Triclade. 

 va, Anterior median intestinal branch ; 

 ha, posterior intestinal branches ; ph, 

 pharynx ; pt, pharyngeal pouch. 



direct from the intestinal aperture ; 1 unpaired, which runs for- 

 ward in the median line and often possesses lateral offshoots which 

 again branch, and 2 paired, which run backward at the sides of the 

 pharyngeal pouch and give off lateral branches towards the exterior. 



C. Rhabdoecelidse. The gastro-canal system is reduced to a simple 

 sac-like or tubular intestine (Fig. 105), which runs in the middle line 

 of the body. This occasionally (especially in the Monotidce) has 

 numerous short lateral diverticula. 



