62 W. BALDWIN SPENCER. 



has been noted by Hoyle in P. protelis, and by Stiles in P. 

 proboscideum. In the mature form the portion of the 

 gland between the duct and the body-wall becomes thinned 

 out^ and thus, just as in the case of the ovary, but not quite to 

 the same extent, the tubes seem to enclose the alimentary 

 canal and the hook-glands. The two vesiculse unite at the 

 anterior end of the testis to form the single chamber, into 

 which open the two ducts from the testis which have been 

 already described. 



In the mature male the nature of the wall of the vesiculse, 

 both in the common part and in the two tubes which pass 

 down to the vasa deferentia on either side, is most unmistak- 

 able. It consists (fig. 37) of (1) an internal layer of cells, 

 which are most evidently glandular in nature {V. S. E.). The 

 outlines are, as a rule, indistinguishable. Internally many 

 of them are swollen out into large globular structures, which 

 have clearly been filled with fluid material to be passed into 

 the lumen of the tube. This fluid will serve for the nourish- 

 ment of the sperm, with which the vesiculse are crowded. It 

 may be noted that only ripe spermatozoa are ever found in the 

 vesiculae. In the inner portion of the layer numerous nuclei 

 are seen, and radial lines indicating doubtless the bases of the 

 cells. Leuckart suggested the glandular nature of these cells 

 in P. taenioides; whilst Stiles in P. proboscideum, refer- 

 ring to this suggestion of Leuckart's, says, " Ich fand hin- 

 gegen, dass diese Schicht sich aus ziemlich flachen Zellen 

 zusammensetzt." Hoyle, in P. protelis, describes them as 

 columnar. Probably in the majority of forms Leuckart's sug- 

 gestion will be found to hold good : certainly this is the case 

 in the form under discussion, where their nature is unmis- 

 takable. (2) External to these cells lies a thin clear layer, the 

 tunica propria of other authors (T. P.). (3) External to this 

 is a strongly developed layer of circularly disposed muscle- 

 fibres {M.V.S.). (4) Most externally is a thick sheath of 

 connective tissue {Ct.) 



(c) Vas Deferens. — This is a short tube on each side of 

 the body with very thick walls, which communicates poste- 



