iv EENAL OKGANS 219 



commence again when pressure is restored. This association of 

 flame-cells with the spaces of the mesenchyme is seen in the more 

 lowly forms in which they occur and it is therefore justifiable to 

 regard it as primitive in spite of the exceptional cases in which 

 flame-cells occur in coelomic cavities. 



The question of the relative antiquity in evolution of coelomic 

 funnel and flame-cell is one which cannot be decided with certainty, 

 depending as it does in turn on the unsolved question as to whether 

 coelome or mesenchyme was evolved first. Looking to the occurrence 

 of mesenchyme in Coelenterates {e.g. the Alcyonarians), in animals 

 in which there is not as yet any closed-off coelome, the balance of 

 probability seems to be on the whole in favour of the flame-cell 

 having originated first, in other words in favour of the original 

 nephridium being of the type called by Goodrich protonepbridium. 

 The fact that existing excretory tubes of this type arise from the 

 ectoderm is also an argument for its antiquity, as it seems natural 

 to suppose that primitively excretory products were got rid of at 

 the outer surface of the body. 



In the primitive ancestral form the genital cells, formed by the 

 lining of the enterocoelic pouches, would reach the exterior through 

 the protostoma or primitive mouth but as evolution proceeded and 

 the coelenteric pouches became separated from the enteron to form a 

 closed coelome 1 another mode of exit would have to be evolved. 

 The natural mode of such exit would be by rupture of the coelomic 

 wall at its weakest spot. Such weak spots would be provided at 

 points where the cavity of the nephridial tube came into proximity 

 with that of the coelome. At such points rupture would take 

 place and the tendency would be for such a temporary rupture, at 

 the time of maturity of the genital cells, to be replaced by a 

 permanent 2 opening from coelome into nephridium. This permanent 

 opening would be the coelomic funnel (coelomostome, nephrostome 

 in the original sense). 



The coelomic funnel, though originally developed to transmit 

 the genital cells, would necessarily also serve as an exit for super- 

 fluous coelomic fluid, and the fluid so transmitted would necessarily 

 serve incidentally to flush out excretory matters passed into the 

 lumen of the tube by the activity of its walls and would thus fulfil 

 the function originally fulfilled by the fluid drawn in by the flame- 

 cell. The function of the flame-cells being in this way otherwise 

 provided for they would tend to disappear. 



The nephridial tube thus came to transmit (1) the reproductive 

 cells and (2) the highly poisonous excretory products. There is, as 

 it appears to the writer, ample evidence that under such circum- 



1 The argument involves as will be seen the assumption that the coelome was in 

 its evolutionary origin enterocoelic. This assumption appears to be justified by the 

 numerous cases in which the coelome so arises in ontogeny. 



2 An actual case where such a temporary rupture, brought about at parturition, 

 has come to be a fixed character of the species and develops independently of 

 mechanical rupture, is seen in the "median vagina" of certain Marsupials. 



