102 



BIOLOGICAL LECTURES. 



the same as far as the outermost cell opening into the nephridi- 

 opore, or the terminal vesicle. We have now studied the 

 structure of this intracellular canal system, but we ought also 



to give an explanation of the cause 

 for its formation. 



In order to gain an idea about 

 the formative cause of these remark- 

 able structures, let us suppose for a 

 while that the receptaculum is con- 

 nected with the exterior by only 

 one cylindrical cell. On the inner 

 surface of this cell granules will be 

 discharged from the receptaculum 

 and carried into the cell, where they 

 are immediately surrounded by vac- 

 uoles (Fig. 21, i). These vacuoles 

 will be equally distributed through 

 the whole mass of the cytoplasm, 

 and where the cell surface is exposed 

 to the exterior some of the vacuoles 



will burst and their contents will be discharged to the outside 

 (Fig. 22, e). The cell is thus partly released from excretory 

 products, but new granules are continually taken up at the 

 inner surface and isolated by fluids, 

 and new vacuoles are emptied at the 

 outer surface. We clearly see that 

 there is thus a continual push coming 

 from the inner surface, and a pull, so 

 to speak, coming from the exterior. 

 These forces produce in the liquid 

 contents of the cell a continuous 

 stream towards the exterior. It is 

 evident that the celerity of this 

 stream is greatest near the outer sur- 

 face of the cell, because the friction 

 is least and there is hardly any resistance. This point of least 

 friction is, therefore, to be considered as a force centre, from 

 which a leading direction is given to the streaming fluids, the 



FIG. 20. 



FIG. 21. 



