378 M. s. PRATT, 



prostatica. The vestibule would, thus, represent the forward end of 

 the uterus. 



I differ from Juel as to the manner in which the fusion has pro- 

 bably taken place. The vestibule has in the the young worms the 

 exact structure of the typical Trematode cirrus and cirrus-sac. I am 

 of the opinion, therefore, that it is the end of the uterus which has 

 dropped back along the ejaculatory duct to a position represented by 

 the posterior end of the vestibule, and that this organ represents the 

 forward end of the ejaculatory duct, i. e. the cirrus and cirrus -sac. 

 I have proof, also, that in the young animal under discussion it may 

 function as a cirrus or penis. In the live worm represented in PI. 25, 

 Fig. 2, which was under the pressure of the cover -glass, I saw the 

 forward portion of the vestibule thrust out far beyond the surface of 

 the body (PI. 25, Fig. 2, p), and spermatozoa emitted from it into 

 the surrounding water. It remained protruded for more than an hour 

 and was finally slowly withdrawn within the body. Looss (1896) has 

 also seen the forward end of the vestibule protruded after the manner 

 of a penis, and is of the opinion that the outer layers of longitudinal 

 and circular muscles of the vestibule with the parenchymatous layer 

 beneath (PI. 27, Fig. 14, out. I. m., out. c. m., and par.) constitute a 

 true cirrus -sac. The tube running through this sac he holds to be 

 the sinus genitalis or genital cloaca. The genital vestibule, thus, ac- 

 cording to him, is made up of the prolonged genital cloaca surrounded 

 by the muscular cirrus-sac. 



The structure of the genital vestibule in the adult, as observed 

 by Juel, differs in certain respects from that of the young worm. 

 In the adult the parenchymatous layer is relatively much thicker and 

 the muscle - layers much less prominent. In fact Juel does not de- 

 scribe inner longitudinal fibres and he describes the parenchymatous 

 layer as being bounded on the outside by longitudinal connective- 

 tissue fibres alone, making no mention of muscle-fibres. The struc- 

 ture of the vestibule as seen in the full-grown adult worm would thus 

 appear to be considerably modified from its condition in the young 

 worm. 



General Considerations. 



On the life-history of appendiculate Distoraes. 

 These worms are simply constructed , digenetic Trematodes with a 

 comparatively simple life-history. The adults are small worms, the 

 largest being under 12 mm long, which have been found in the in- 



