THE PHYSIOLOGY OF EXCRETION. 



IOI 



In a few of the cells following those last described the cutic- 

 ula of the central canal is reenforced by a network of fibres 

 which is first irregular, but soon assumes a definite arrange- 

 ment. This arrangement is as follows: 



Imbedded in the cuticula we find thick threads which form 

 a perfect ring around the canal and stain deeper than the cyto- 

 plasmic threads. These rings are placed at right angles to the 

 axis of the canal and lie at regular short intervals from each other; 



FIG. 14. 



FIG. 15. 



FIG. 16. 



FIG. 17. 



FIG. 1 8. 



FIG. 19. 



they are studded with dark-staining coarse knobs (/), which are 

 likewise placed at equal distances from each other. Between 

 the rings there are anastomoses in the form of fine cytoplasmic 

 threads which run from one of these knobs to other knobs of 

 the next ring (Fig. 20, m). I have not the least doubt but that 

 this structure represents a musculature of the cell, that the 

 ring fibres are contractile, and that by their contraction a 

 peristalsis ensues, which hastens the discharge of the contents 

 of the canal to the exterior. The side canals have entirely 

 disappeared in these cells, and the muscular structure remains 



