CH. V] 



EXCRETORY SYSTEM 



93 



in all the animals which we have so far studied excretion seems to 

 be performed by each cell for itself. In a sense this is true of all 

 animals all protoplasm must be continually producing excreta so 

 long as it is living and these excreta must somehow be got rid of. 

 So long as protoplasm is arranged (as in most Coelenterata) in the 

 form of thin layers of cells lining tubes, excreta are easily voided 

 into the cavities of the tubes, or in the case of ectodermal cells 

 directly to the exterior. 



When however we have thicker masses of cells such as are met 

 with in the parenchyma of Platyhelminthes, then their excreta must 

 pass into the fluid in which they are 

 bathed, i.e. the fluid that fills the space 

 between ectoderm and endoderm, into 

 which the cells forming the parenchyma 

 pass. As the percentage of excreta in 

 this fluid rises it tends to become 

 poisonous to the cells unless the ex- 

 creta are in some way crystallised out 

 from it. This then is the function of 

 an excretory organ it precipitates the 

 excreta in the body-fluids in its cyto- 

 plasm. The precipitated excreta may 

 be retained in the excretory cell as 

 insoluble granules till it dies and drops 

 off, or they may be redissolved and cast 

 forth as an external secretion by that 

 cell. The ectoderm seems to have been 

 the original excretory organ, and the 

 first distinct excretory organs which we 

 encounter in Platyhelminthes for the 

 first time appear to arise as tubular 

 ingrowths of ectoderm into the paren- 

 chyma. These ingrowths branch re- 

 peatedly and each branch terminates in 

 a peculiar cell known as aflame-cell or 

 solenocyte. "Such a cell is hollowed 

 out by an extension of the excretory 

 tube which ends blindly within it and is 



lined by a cuticular substance. From the blind end there grow 

 out one or more flagella which project into the tube and by their 

 vibration suggest the flickering of a flame. Water passes through 



FIG. 40. Two flame-cells or 

 solenocytes from the ne- 

 phridium of an Annelid 

 worm. (After Goodrich.) 



1. Flagellum of the flame- 

 cell. 2. Branch of the 

 nephridial tube leading into 

 the flame-cell. 3. Nucleus 

 of flame-cell. 4. Main 

 tube of the nephridium. 



