THE PHYSIOLOGY OF EXCRETION. 103 



same as the centre of gravitation gives direction to moving 

 particles. As a result of this we find the formation of a single 

 canal in the outer part of the cell, into which a number of side 

 canals (c) open radially (Fig. 23). In the innermost part of the 

 cell we find isolated vacuoles, because new granules are con- 

 tinually taken up and isolated. These vacuoles flow together 

 in different directions, and only in the lower part of the cell a 

 regular arrangement takes place. 



Let us now suppose that this cell divides into two, three, or 

 more daughter cells, and we shall get a structure similar to 

 that of a row of adult nephridial cells. The innermost cells {i) 





-t 



Fig. 22. 



Fig. 23. 



are vacuolated, the next pierced by irregular canals {c), the next 

 with radial side canals (c), and the outer cells with a single cen- 

 tral canal (Fig. 3). Let us go a step further. We know that 

 every stimulus is, from the side of the bioplasm, followed by a 

 reaction. The more intense the stimulus, the more energetic 

 the reaction. We easily see that the excretory granules within 

 the isolating vacuoles cannot effect any appreciable stimulus 

 upon the surrounding cytoplasm, but the case is quite different 

 where these vacuoles flow together and form a system of canals, 

 within which a continuous stream of fluid flows. This stream 

 is quickest near the outer opening of the cell, and granules will 

 be thrown against the surrounding cytoplasm and stimulate it. 

 This is exactly similar to what happens in a river. The erosion 

 by the river works continually upwards, thus forming valleys, 

 mountain gorges, and passes. A slow, irregular stream of fluid 

 flows in the anastomosing, irregular vacuole canals. The 



