A NEW SPECIES OF PENTASTOMUM. 183 



After a course of about 001 mm. this vesicula seminalis becomes divided 

 into two by a septum which rises from its ventral surface (fig. 7), and very 

 shortly it divides into two quite distinct tubes (figs. 8 and 9). This junction 

 between the two vesiculse seminales is of considerable morphological interest, 

 because it is the first trace of a condition which is carried further in P. pro- 

 boscideum, and reaches its fullest expression in the large median sac found in 

 L. tcenioides (cf. Leuckart, loc. tit, Tab. ii. figs. 9 and 10). 



At this point the dorsal mesentery splits, and a portion accompanies each 

 tube. 



The two branches of the vesicula seminalis now diverge from each other, 

 and passing round the intestine, become united with the copulatory apparatus 

 which lies upon its ventral surface ; as they separate they first of all lose their 

 mesenteric connection with the dorsal body- wall, although the rudiment of the 

 mesentery is still to be seen after they have become detached from it. In their 

 further course they approach the hook-glands, which they gradually traverse, 

 passing through the middle of the gland and not between it and the intestine. 



The rudimentary mesentery now demands a moment's notice ; it passes 

 forwards, becoming slightly wider, and just at the point where the vesicula 

 seminalis emerges from the hook-gland, it becomes connected with this latter ; 

 so that now each hook-gland is supported by two mesenteries, one which it has 

 had throughout the whole length of the body, and another which it has derived 

 from the genital organs. 



In this connection it may be well to say a word or two with respect to the 

 size and structure of the vesicula seminalis. It is about 2 mm. long, and 

 of a regular oval section, its greater and lesser diameters being 0*178 mm., 

 and 0*1 mm. respectively, being slightly larger about the middle of its length. 

 Its wall is made up of two layers of about equal thickness (0*02 mm.); the 

 outer of which consists of a mass of cells very similar to those mentioned at 

 its commencement, but rather more irregular in outline, and varying consider- 

 ably in size, and provided with distinct ovoid nuclei. The inner coat consists 

 of a single layer of elongated columnar epithelial cells, also nucleated and 

 separated from each other by an undulating boundary line. 



The vesicula seminalis now lies free in the body-cavity for a short distance, 

 and then becomes attached to the vas deferens,, which will be presently 

 described. 



At this point one of the most remarkable peculiarities in the organisation of 

 the animal comes into notice. The lumen of the vesicula seminalis becomes 

 excluded, and is not continuous with that of the vas deferens. 



This last is a tube regularly elliptical in section, of sharply-defined even 

 contour and compact walls ; applied to the dorsal wall of this is a mass, oval in 

 section, of much looser tissue, which is in fact the wall of the vesicula seminalis, 



