THE PHYSIOLOGY OF EXCRETION. 97 



pebbles into a vessel full of water. As soon as the vessel is 

 entirely filled with pebbles nearly all the water has overflowed. 

 As to the question how the granules get into the nephridium 

 I have not any definite idea. It is possible that, after the recep- 

 taculum is entirely filled with solid waste products, these may 

 effect a stimulus upon the wall of the receptaculum and, with 

 the aid of a chemotropism towards the nephridial cells, might 

 be forced through small spaces between the connective tissue 

 cells of the wall into that neighboring nephridial cell which is 

 in direct contact with the receptaculum. Sometimes the recep- 

 taculum even overlaps a great part of this innermost ne- 

 phridial cell. 



This structure of the funnel fulfills the double end, firstly, of 

 preventing nutritive blood from being wasted, and, secondly, of 

 preventing the topmost nephridial cell from being overloaded 

 with foreign matters, to take care of which would be an impos- 

 sible task for the cell. 



The further fate of the excretory granules before they reach 

 the exterior is highly interesting, and can only be determined 

 by a very careful study of the structure of nephridial cells 

 from different parts of the nephridium. 



In the following discussion I shall use the term inner cells 

 for those cells of the nephridium which are near the funnel, 

 and outer cells for those which are nearer the nephridiopore. 



The structure of the two or three innermost cells of the 

 nephridial row is as follows : The nucleus is very irregular in 

 shape and is surrounded by a thin membrane, which is broken 

 in places. The cytoplasm consists of a beautiful threadwork 

 of very distinct anastomosing threads, round which the micro- 

 somes cluster. Besides the cytolymph, we find between the 

 meshes a great number of vacuoles (v) in the cytoplasm, and in 

 these vacuoles (which are filled with a watery fluid) we discern 

 small granules, which are no other than the excretory granules. 

 The vacuoles vary considerably in size. Here, again, is a mani- 

 festation of the isolability of cytoplasm with regard to foreign 

 solid particles. The cytoplasm is stimulated by these granules 

 and secretes around them an indifferent fluid (Fig. 7). In a 

 number of the following cells we notice new structures which 



