STUDIES ON* THE ECTOPAEASITIC TREMATODES OF JAPAN. 9 \ 



each a single nucleolus, as in Microcotijle cliiri (PL V, fig. 4), 

 M. sciaeiue (PI. VI, fig. 2), and Onchocotijle spinacis (PI. XVI, 

 fig. 8). It seems to me therefore clear that -the structureless membrane 

 of the vas deferens of most species is to be regarded as the transform- 

 ed product of the originally cellular epithelium ; and this becomes the 

 more px'obable when we see that nuclei are present in some parts of 

 the uterus but are wholly absent from others, as will be described later 

 on. In' those species in which the wall of the vas deferens consists 

 only of a structureless membrane there is often a coarsely granular 

 layer on the inner surface of the wall, which will be described pre- 

 sently in treating of the prostate gland. In most species the 

 vas deferens is wholly destitute of any musculature ; but in some, as 

 in Microcotyle sciaeiue (PI. VI, fig. 2) and Hexacotijle (PL XII, fig. 5), 

 it is provided with a single layer of circular fibres. 



In Monocotyle there is a peculiar organ around the vas deferens at 

 a short distance from where this opens outwards (PL XVII, fig. 1 

 & PL XVIII, fig. 3, hill. ej.). It is spherical in shape, is hollow, 

 and is traversed by the vas defei'ens. The wall of this organ is very 

 thick, and consists of connective tissue fibres which are all arranged 

 radially ; it is bounded both internally and externally by a structure- 

 less membrane ; but the external membrane is incomplete for a short 

 space on the dorsal side, and here the substance of the wall is directly 

 continuous with the surrounding mesenchyma, — the one passing 

 gradually into the other — thus showing that both are of the same 

 nature. On the surface of the internal limiting membrane there is a 

 thin granular layer ; and just externally to the same membrane there 

 is a layer of circular muscular fibres (PL XVIII, fig. 3, hid. ej.). The 

 only use that I can attribute to this organ is to eject the sperm, 

 and I shall therefore call it hulhus ejaculatorius. Around it there 

 is a circular canal, the plane of which coincides with that of 



