94 ENTOZOA. 



succeeded -by a single vas deferens, terminating in the cloacal cavity. 

 The bifurcate penis lies immediately in front, beiug inclosed in a 

 separate sbeatli capable of partial protrusion at tbe cloacal outlet. 

 In tlie female tbe essential sexual organs form two long uterine or 

 ovarian tubes filling the interior of the body and passing downwards 

 towards the tail, where they combiue to form a common broad ovi- 

 duct, which latter opens by a constricted orifice directly in front of 

 the anus (Fig. 3, Plate VII.) In front of the vulva and directly in 

 contact with the anterior border of the vaginal or narrow portion of 

 the oviduct there is placed a large spherical vesicle which projects 

 externally on the right side of the tail. The natiu-e of this develop- 

 ment is not very obvious, but it may possibly assist in the sexual 

 act by offering to the tentacular-lobed appendages of the male a 

 point of resistance for the full development of their clasping action. 

 All the species of this genus reproduce viviparously. If the 

 worms are examined in the fresh state the young may occasionally 

 be seen to escape by the vaginal orifice. Yan Beneden noticed 

 this phenomenon in Prosthecosacter inflexus, and the same thing was 

 observed by Mr. Busk in P. convohitus. In the instance here illus- 

 trated one of the embryos is seen in the act of emerging, its caudal 

 extremity being stiU lodged within the vulva of the parent (Fig. 3, 

 Plate VII.) In the fresh worm one may also see, under the magnifying 

 glass, numerous young coUed together within the oviduct (Fig. 2, 

 Plate VII.), and the last-named organ widens out into a capacious 

 sac at a little distance from the end of the tail (Fig. 3). Speaking 

 roughly, the embryos measure about the 3-^ of an inch in length, 

 by the ^ of an inch in breadth. Higher up within the uterine 

 and ovarian ducts, the ova may be seen in all stages of develop- 

 ment, according to the particular region of the tube under 

 examination (Fig. 4, Plate VII.) In their full-grown condition, 

 the eggs have a longitudinal diameter of i^" by a transversal 

 measurement of about ^ of an inch. So far as I am aware, 

 the essential act of fecundation and the phenomena of the 

 development of the ova, as they occur prior to impregnation, have 



