THE ANATOMY OF PENTASTOMUM TERETIUSCULUM. 53 



riorly with the vesicula seminalis, and anteriorly with a short 

 tube called by Stiles the '' Cirruszwiebel " (figs. 6 and 8). 



Together with the remaining parts of the male organs to be 

 afterwards described^ it stands in marked contrast to the 

 testis and vesiculse in that it possesses a cuticular lining to 

 its duct. It is therefore to be regarded as distinct in origin 

 from these two structures, and as formed from an invagina- 

 tion of epiblast which has united with and opened into the 

 vesiculae. It is interesting to note in connection with this, 

 that in the specimens of P. protelis examined by Hoyle, and 

 which were hence presumably immature, the lumen of the 

 vesicula was not continuous with that of the vas deferens. 

 The two sets of structures are independent in origin, one 

 being formed from mesoblast and the other from an epiblastic 

 invagination, and the union of the spaces contained in the two 

 is apparently in P. protelis only established comparatively 

 late in development. 



The duct of the vas deferens is formed of a layer of very 

 minute cells with internally a distinct cuticular lining. This 

 duct (fig. 47, V. D.) is enclosed in a thick coat of cells 

 precisely similar in structure to those around the duct which 

 in the female organ passes from the union of the oviducts to the 

 spermatheca on either side. Each cell is of fair size (-06 mm.), 

 and contains a darkly staining nucleus, the outline of which 

 is often indistinct, whilst at others a distinct nuclear mem- 

 brane can be distinguished. The protoplasm is markedly 

 reticulate (in preserved specimens), and the outlines of the 

 cells, which are irregular in shape and vary much in size, are 

 distinct. No muscular elements are distinguishable. 



Ventrally to the union of this with the vesicula, it is con- 

 nected with a tube which may be regarded as a special back- 

 ward running diverticulum of the vas deferens, and which will 

 now be described. 



{d) Ejaculatory Duct. — This forms one of the most 

 prominent features in the male organ, and there can be no 

 doubt that its name (Propulsionsschlauch), suggested by 

 Leuckart, is indicative of its function. It arises on each side 



