TUBIFEX. 367 



the vas deferens is a gradual one, and this transitional 

 part of the tube is worth further examination. As has 

 been already noticed, the lumen remains of the same size 

 throughout. The wall, however, thickens gradually. 

 The cells of which it is composed become somewhat larger, 

 and are a little further apart from one another, but their 

 boundaries are quite distinct (PI. V, fig. 21). The 

 matrix increases in quantity, it is developed between 

 adjacent cells and also in a thick layer outside them. The 

 fibrillae are quite distinct, and very numerous even in 

 this region. For a short distance the cells of the second 

 part of the tube are ciliated, but there are no annular 

 rings. As soon as the rings appear the cilia are lost, and 

 at this stage also the cell boundaries become less distinct. 



The vas deferens of quite a young worm is ciliated 

 throughout, and the wall of the second part of the tube is 

 only very slightly thicker than that of the first. That is. 

 the matrix is only present in small quantities at first, 

 and there are but slight indications of the annular rings. 



It seems strange that so little notice has been taken 

 of the complicated structure of the vas deferens in this 

 worm, and, possibly, in many others. Beddard, as well 

 as stating that the tube is ciliated throughout, adds in 

 another place that the wall in Oligochaeta as a whole is 

 usually composed of an inner ciliated epithelium and an 

 outer peritoneal covering. He seems to consider it a very 

 exceptional condition for there to be any muscular 

 elements at all, and he quotes Eudrilus (an earthworm) 

 as his only example. Nasse (1882) seems to have been the 

 first and only observer to note the essential points in the 

 structure of the vas deferens of Tub if ex, but his descrip- 

 tion gives one the impression thai he was uncertain on 

 some points, such as the actual form of the cells, whether 

 they were annular or only spindle-shaped, and whether an 



Y 



