vi NEPHRIDIA 341 



the tubule is situated. The nephrostome opens into a 

 long and slender, transparent part of the tube, lined with 

 ciliated cells in part of its course and extending along the 

 first and second loops (b) this part is succeeded by a 

 wider, ciliated portion in the second loop (<:), which com- 

 municates with a still wider portion (d) lined by granular, 

 non-ciliated, glandular cells, also lying in the second 

 loop ; the glandular portion opens into a much wider 

 muscular part of the tube (e), which constitutes the third 

 loop and communicates with the exterior by a small pore 

 the nephridiopore near the outer seta of the inner couple. 

 The muscular part is lined by an epithelium, while the rest 

 of the nephridium is formed of a single row of hollowed 

 cells, set end to end, like a series of drain-pipes, so that their 

 cavity is intraedlular, and not intercellular. 



Thus the nephridia, which are abundantly supplied with 

 blood-vessels, are lined in part by gland-cells and in part 

 by cilia which work towards the exterior. Water and 

 nitrogenous waste from all parts of the body pass by diffu- 

 sion into the blood and are conveyed to the nephridia, 

 the gland-cells of which withdraw the waste products and 

 pass them into the cavities of the tubes, whence they are 

 finally discharged from the body. The granular yellow 

 ccelomic cells on the wall of the intestine also appear to 

 contain excretory products, 1 which become set free in the 

 body-cavity and are thence got rid of by means of the 

 nephridia. It will be noticed that a certain amount of loss 

 of the ccelomic fluid must take place through the dorsal 

 pores as well as through the nephridia. 



In discussing the hydroid polypes we found that one of 

 the most important points of difference between the loco- 



1 It is probable that in primitive forms the whole ccelomic epithelium 

 was excretory in function. 



