TUBIFEX. 379 



vary considerably according to the condition of the worm — 

 in a mature worm the duct is usually somewhat longer 

 than the pouch, but in the younger condition they are 

 more nearly equal. Typically, the spermatheca lies 

 entirely in segment 10, but about the time of copulation, 

 and certainly when the spermatozoa are to be found in 

 these organs, they enlarge considerably and may be either 

 coiled round in segment 10 or extend into segment 11, 

 or even more posteriorly into segment 12 (PL IV, fig. 16, 

 sp.). In the latter case there is a sharp bend in the 

 spermatheca, and the duct runs forwards again parallel 

 to the pouch in order to open to the exterior on segment 10. 

 The pouch is quite simple, although it is so voluminous, 

 and is devoid of diverticula ; neither has it any connection 

 with the alimentary canal, as is the case in certain 

 Enchytraeidae. 



The structure of the wall of the spermatheca differs 

 considerably in different parts. The wall of the pouch is 

 thin, but three layers are recognisable. Internally there 

 is a single layer of epithelial cells which seem to be of two 

 kinds. Certain of the cells are squarish in shape and have 

 very large nuclei, which are oval or rounded in shape, 

 and usually include one principal and one or more 

 accessory nucleoli. These cells are probably glandular in 

 nature. Between these are smaller, narrow cells, whose 

 nuclei are much less conspicuous, and which we may term 

 interstitial cells. The wall of the pouch is strengthened 

 by the presence of a few scattered muscle fibres placed 

 parallel to the longitudinal axis of the spermatheca and 

 outside the layer of epithelial cells. Outside this again, 

 the wall is covered by a single layer of flattened peritoneal 

 cells, the nuclei of which arc oval in shape and form the 

 most conspicuous part of the cells. 



The duct when seen in transverse section is circular 



