308 W. BLAXLAND BENHAM. 



Frequently in a section, or even in several consecutive 

 sections^ no nucleus appears ; then perhaps a section ^vill show 

 only one, or others show two, and very rarely three (fig. 16) ; 

 these nuclei are quite similar to those in other parts of the 

 nephridium — oval, with a nucleolus ; and they appear to be 

 embedded in a finely granular though very thin layer of proto- 

 plasm, showing a feebly reticular structure (fig. 16, ep.); in 

 which, however, I can detect no cell boundaries. 



Immediately outside this layer of protoplasm comes the 

 muscular coat {muse), which consists of cells arranged in 

 various directions (fig. 16) ; some are longitudinal, others cir- 

 cular ; still others are oblique, in relation to the long axis of 

 the duct. These muscles appear also to be embedded in finely 

 granular protoplasm (cep.), in which small round nuclei are 

 scattered. It is difiicult to make out a continuous layer of 

 flattened coelomic epithelial cells outside the muscular coat, 

 either in section or in nephridia treated with nitrate of silver. 

 From the appearance presented in many sections this granular 

 protoplasm seems to represent the vesicular connective-tissue 

 cells plus ccelomic epithelium of the other parts of the tube. 



The muscular duct penetrates the body-wall, the muscles 

 appearing to be continuous with those of the circular layer ; 

 a slight invagination of the epidermis meets the nephridial 

 lumen, and puts the latter into communication with the 

 exterior. 



With a view to elucidating the character of the epithelium 

 lining this part of the tube I have examined sections through 

 other earthworms, and amongst these Microchaeta rappi, in 

 which the muscular duct is of enormous size (see my paper 

 [8], pi. xvi, fig. 21). In this worm the lining of this region of 

 the nephridium consists of very numerous vesicular cells, quite 

 unlike what we have in Lumbricus; they are small, with 

 distinct boundaries, and appear to be but loosely attached to 

 the wall. This layer of cells can be traced through the body- 

 wall up to the circular layer of muscles, where it meets the 

 epidermic pit. I think, therefore, that we may conclude that 

 in Lumbricus, too, the lumen of this region is intercellular, 



