104 



BIOLOGICAL LECTURES. 



granules suspended in this stream will lightly graze the sur- 

 rounding protoplasm, a stimulus sufficient for the secretion of 

 a cuticula. Where the stream is quicker the friction is greater, 

 and meshes of cytoplasmic threads are formed in the cuticula 

 for a reinforcement. Near the outer opening the stream is 

 very rapid, and here the cytoplasmic threads are regularly 

 arranged in rings and transformed into contractile substances, 





c 



FIG. 3. 



rings which contract as soon as a granule floating in the canal 

 is hurled against them. This contraction only accelerates the 

 stream, and thus we understand that this muscular structure 

 also progresses from the outer cells inwards. The thick granules 

 which stud the rings I regard as the direct receivers and trans- 

 mitters of the stimuli, the anastomosing threads as the sensory 

 conductors, facilitating by their activity a coordinated peristal- 

 sis. By the aid of this complicated mechanism within the 

 nephridial cells, the excretory granules are finally discharged 

 from the body. It is evident that these excretory granules are 

 not the only waste products, but that the fluid contained in the 



