vi XEPHRIDIA 41 



of 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 (c), which communicates 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 

 intracellular, 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 diffusion 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 coelomic 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 coelomic 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 



1 It is probable that in primitive forms the whole ccelomic 

 epithelium was excretory in function. 



