THE TURBELLARIA. 



159 



externally by a median pore on the oral face of the body, the 

 male aperture being posterior to the female. The margins of 

 the male aperture are produced into a curved prominence, the 

 penis. 



Tiiose Tarbellaria which resemble Jfacrostomum in having 

 a straight, simple digestive cavity, are termed Hhabdocoela. 

 They, for the most part, possess a buccal proboscis, which is 

 capable of being protruded from, or retracted into a chamber 



>Sfe 



sfj.-j:', . 



m 





Pig. 32. — Opisthomum (after Schnlze).— a, central nervous system ; ramifications of 



their hard shells 



formed by the walls of the circum-oral region of the body 

 (Fig. 32, c). 



In some (e. g., Prostomum) the anterior end of the body is 



