TUBIFEX. 381 



considerably than the radius of the cavity of the duct, so 

 that they do not obstruct the passage much. 



The muscular layer is much more powerfully 

 developed around the duct than on the pouch. The 

 muscles form a double layer, those on the inside being 

 arranged circularly, while outside this is a very definite 

 sheath of longitudinal muscles. These appear to be 

 continuous with those on the pouch, the circular muscles 

 being interposed between them and the ciliated 

 epithelium. The cells of the peritoneum covering the 

 duct are somewhat different from those investing the 

 pouch. They are no longer flattened, but squarish in 

 outline, and form a rather more conspicuous layer than 

 that of the pouch (PI. V, fig. 23, pt.). The nuclei are 

 usually situated near the middle of the cell, the contents 

 of which are very granular. 



The terminal portion of the duct of the spermatheca 

 is protrusible, and when it is protruded the duct narrows 

 considerably near the end and leads to the exterior by a 

 very narrow but straight passage. Near the spernia- 

 thecal pore (PI. IV, fig. 16, sp. p.) the ciliated epithelium 

 ceases, and that which lines the narrow passage leading 

 to the exterior is similar in structure to that forming the 

 epidermis of the body wall, with which it is directly 

 continuous. 



The muscular layer extends to the end of the duct, 

 where the muscles become connected with those of the 

 body wall. When the terminal portion of the duct is 

 retracted, the passage to the exterior is no longer 

 straight, but its wall is folded back in the form of a 

 cone or arrow-head. 



