THE PLATYHELMIA 



and that each became elongated and effected a communication 

 with the exterior, and it has even been suggested that such cells 

 are derived from unicellular epidermal glands which have gradu- 

 ally sunk into the parenchyma, retaining their communication 

 with the exterior (on the other hand, similar flame cells have been 

 noted amongst the intestinal epithelium). As these isolated cells 

 became more numerous, with the increase in the size of the animal 

 there would be, as in other analogous instances, a tendency to form 

 a common collecting canal which would then replace the numerous 



J 



Fia. I. Diagram of the Structure of the Platyhelminth Excretory System. 



1. A portion of the system of canals, a, the main canal or duct, receiving numerous 

 secondary canals (h) ; c, flame cells or terminal cells of the capillary vessels ; d, nucleus in the 

 wall of the canals ; c, tuft of cilia or "flame," arising from the side of the canal, in the neigh- 

 bourhood of a nucleus. 



2. A flame cell (somewhat diagrammatic), o, nucleus ; 6, excretory globules in the cyto- 

 plasm ; c, the "flame"; rf, protoplasmic processes of the cell which extend amongst the 

 parenchymal cells, and are possibly in connection therewith ; e, the canaliculated portion of the 

 cell which communicates with the neighbouring excretory tubules. 



isolated apertures. Nevertheless, in many existing forms, such 

 scattered or regularly arranged, isolated apertures exist either in 

 the absence of, or in addition to, the collecting canal and its pore. 



The metenteron was a simple sac, having a single opening to 

 the exterior, which served both for inception of food and ejection 

 of faeces. This mouth was probably somewhere between the 

 centre and the anterior end of the oval body, as it is in many of 

 the Turbellaria. The surface in which the mouth is situated is 

 the ventral surface (Fig. II.). 



The nervous system, like the muscular, had separated from the 

 epidermis and had taken on a much more definite form than in the 



