On Dissotrema papillatum. 9 



wards the right side of the body; and from the right side of the 

 left testis a similar duct proceeds towards the right and obliquely 

 forwards, and uniting with its fellow opens into the vas deferens 

 (fig. 4). This is a large duct beginning with a sudden expansion 

 where it joins the vasa efferentia and proceeding transversely across 

 the body with a light posteriorly convex curvature, opens directly 

 in front of the left testis into the next portion, which may be called 

 the seminal vesicle. The wall of the vas deferens is formed by a 

 thin membrane and is destitute of muscle fibres. The left vas 

 efferens runs parallel to the vas deferens, the former on the dorsal 

 side of the latter. 



The seminal vesicle is a large spindle-shaped organ communi- 

 cating with the vas deferens on the left side of the body by a con- 

 stricted opening and lying transversely across the body directly in 

 front of the vas deferens and parallel to it. At its right end it un- 

 dergoes a narrow constriction forming a circular valve projecting 

 into the next portion, the ejaculatory duct. The wall of the semi- 

 nal vesicle is formed by a thin membrane exactly like that of the 

 vas deferens, but in addition there is a layer of well developed cir- 

 cular muscle fibres. The seminal vesicle lies ventrally to the other 

 genital organs found at the same level, and is like the vas deferens 

 almost always filled with sperm. The ejaculatory duct is a canal of 

 some length, whose calibre varies a great deal in different examples 

 according to its sperm contents; in one of the examples that 

 came under observation it was a slender canal and entirely empty. 

 Usually however it is more or less enlarged and runs from where it 

 joins the seminal vesicle forwards and slightly towards the left, and 

 entering the penis sac opens near the centre of the latter into the 

 penis. Its wall consists of a flattened or cubical epithelium resting 

 on a basement membrane and containing well stained nuclei, 

 followed by a layer of circular muscle fibres, on the outside of which 

 there is a layer of strong longitudinal ones. The muscular coat is 

 relatively thick, being about 8//, and consequently the wall of the 

 ejaculatory duct stands out quite distinctly even in total mounts. 

 That part of the ejaculatory duct which lies outside the penis sac is 

 surrounded on all sides by the prostate glands, conspicuously well 



