THE EARTHWORM 



25 



certain grooved cells which first run forwards then turn 

 sharply backwards, and the groove is converted into a closed 

 tube, which is continued backwards as the narrow ciliated tube 

 passing through the septum. The actual opening of the 

 nephrostome is a crescentic slit lying between the grooved 

 cells and the inner edge of the central cell. (Fig. 5, C.) 



The post-septal part of the nephridium consists of — (i) a 



Fig- 5 



A. Diagram of a nephridium of the earthworm, m.j, nephrostome ; j, septum; 

 ct, narrow ciliated tube ; mi, middle tube ; if/, wide glandular non-ciliated 

 tube; m.d, muscular duct. B. A portion of A enlarged to show the 

 passage from the middle tube to the glandular tube. C. A nephrostome 

 enlarged, m, marginal cells ; c.s, central cell ; £:c, grooved cells. (After 

 Benham.) 



long narrow ciliated tube continuous with the narrow pre- 

 septal tube. This tube is composed throughout of hollow 

 cells joined end to end like so many drain-pipes, and is ciliated 

 in certain parts of its course. (2) A short middle tube, 

 brown in colour and ciliated internally throughout its extent ; 

 (3) a wide tube which is not ciliated; (4) a relatively short 

 muscular tube or duct which opens to the exterior. The 

 three first sections are twined round each other in the manner 

 shown in fig. 5 to form two loops, the muscular duct forming 

 another loop separate from the other two. Calling the 



Digitized by Microsoft® 



