366 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



They are arranged very regularly, originating at the 

 inner edge of the cell and radiating out to the periphery. 

 These fibrillae are so numerous that there is but little of 

 the normal cytoplasm remaining. Owing to the large 

 number of these fibrillae which are present, and to the 

 consequent disappearance of the cytoplasm of the cells, 

 the cell boundaries become very indistinct, and are barely 

 recognisable. The nuclei, however, persist in their 

 original position, and indicate, sufficiently clearly, what 

 was the arrangement of the cells (PL TV, fig. 20b). 



In the mature worm these cells are not ciliated, but 

 the inner edge of each is considerably thickened and 

 strengthened by the deposition of a secretion resembling 

 cuticle in appearance and staining reactions. Since the 

 inner edge of each cell is thus thickened, the secretion has 

 the form of a series of circular bands arranged extremely 

 regularly throughout the length of the tube (PI. IV, 

 fig. 20b, an.r.). It is extremely difficult to decide what 

 is the exact nature of the substance composing these rings. 

 Its position in relation to the cell and to the lumen of the 

 duct suggests a chitinous or cuticular secretion, but there 

 is no doubt that it is capable of contraction, which 

 suggests that it is muscular in nature. These circular 

 bands convert this part of the vas deferens into a much 

 more solid structure than it would otherwise be, and also 

 help to keep the lumen open. Outside the epithelial layer 

 is a single layer of longitudinal muscle fibres arranged 

 similarly to those of the first part of the tube. They are 

 continuous, at the end of the vas deferens, with those 

 which surround the spermiducal gland. A peritoneal 

 covering invests the vas deferens in this part also, and its 

 cells are flattened, forming a single layer, and provided 

 with oval nuclei. 



The change from the narrower to the wider part of 



