262 INVERTEBRATE PHYSIOLOGY 



An assumption of filtration into closed nephridia has been made but 

 proof of this assumption is lacking. Neglecting the "blood filters" of Bahl, 

 may there be another source of filtration pressure ? In 1899 Goodrich made 

 the suggestion that solenocytes may have the same function as Malpighian 

 corpuscles of the vertebrate kidney and this suggestion proved useful ; for 

 water has been seen to accumulate in the contractile bladders of animals 

 possessing protonephridia, water apparently derived from the protone- 

 phridia. But Goodrich speaks for the conservative view of the origin of 

 such fluid when he remarks : " . , . the chief function of the 'flame' is to drive 

 the fluid passing by osmosis through the thin wall of the chambers down 

 the canal towards the nephridiopore." This view does not explain satis- 

 factorily the appearance of the fluid in the nephridium. If, in accordance 

 with the idea expressed by Goodrich, the fluid accumulates by osmosis 

 we have to search for osmotically active substances in the lumen of the 

 nephridium. Salts could be secreted into the lumen by active transport, 

 there to be diluted by water of osmosis. Such a material might well be a 

 potassium salt which could be reabsorbed and reutilized lower in the canal. 

 Ramsay (1954) observed the potassium turnover in the Malpighian cor- 

 puscle of insects, and set up a deliberate test of this hypothesis ; he was 

 convinced that the results of his experiments could not support such a 

 mechanism. 



The tube surrounding the flame of the ordinary nephridium and in par- 

 ticular the tube of its derivative, the solenocyte, does not appear from its 

 very thin nature to be particularly suited to a process of secretion, but is 

 very well adapted to a process of filtration. The problem of finding a force 

 responsible for filtration remains and leads us to the beating of the flame 

 or of the flagellum. Carter ( 1940) examined and rejected the idea that the 

 flame could exert enough pressure to overcome the colloid osmotic pressure 

 of the fluid bathing the flame cells. Bahl (1945) argues that the pressure 

 might be sufficient : "In the closed integumentary and pharyngeal nephridia 

 the movement of the cilia in the ciliated tracts probably sets up a slight 

 pressure which is enough to draw liquid by a process of filtration from 

 the blood and coelomic fluid, through the exceedingly thin walls of the 

 nephridium into the lumen of the intracellular canal." Pantin (1947) re- 

 news the discussion in the light of his findings that the activity of the flame 

 cells is increased by the uptake of fluid in the terrestrial nemertine worm 

 Geoncmertcs dcndyi, and in the light of the visual observation of oscilla- 

 tions at the base of the cell set up by the action of the flame. 



It is most attractive to view the flame cells and solenocytes in this way 

 as sources of filtration pressure. If this interpretation is correct, the wide- 

 spread importance of the process in higher phyla becomes interpretable as 

 a natural succession to the filtration process so uniformly present in the 



