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. 



Those Tarhellaria which resemble 3facrostommn in having 

 a straight, simple digestive cavity, are termed Ukabdocoela. 

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

 capable of being protruded from, or retracted into a chamber 



§^^^ 



s-^: 



Fig. S2.—Opisthomum (aftp.r Schulze).— a, central nerrous system ; ramifications of 

 the water- vessels are seen close to it; b, mouth; c, proboscis; d, testes; e, vasa 

 deferentia; /, vesicula seminalis ; g, penis ; A, sexual aperture ; i, vatrina ; k, sper- 

 raatheca ; L germarium; m, vitellarium ; n, uterus with two ova inclosed within^ 

 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 



