220 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



stances the tendency of subsequent evolution would be to separate 

 from one another the paths to the exterior of the genital cells and 

 of the poisonous excretory products respectively. It might fairly be 

 anticipated for physiological reasons that there would be such a 

 tendency but that it actually exists is demonstrated by the facts of 

 comparative anatomy and embryology. Over and over again we 

 find cases where such separation has undoubtedly come about. For 

 example in the evolution of the Gasteropoda the right nephridium 

 has lost its excretory function and come to be merely a genital duct. 

 In Vertebrates there are several familiar examples of parts of the 

 renal system which have to do with transmitting genital cells 

 becoming separated off from those which retain a renal function. 1 



The present writer then believes the balance of probability to be 

 in favour of the evolutionary origin of the type of nephridial tube 

 commonly met with in coelomate animals, possessing a coelomic 

 funnel or nephrostome at its inner end, having come about in the 

 manner outlined above. The essential difference between the view 

 here outlined and that developed by Goodrich is' that it rejects the 

 idea that evolution has brought about a more and more intimate 

 connexion between originally independent genital funnel and 

 nephridial tube as opposed to physiological probability. On the 

 contrary it regards the funnel as having opened into the tube at 

 the time of its first appearance, the progress of subsequent evolution 

 having been in the direction of separating genital funnel and 

 nephridial tube and not of uniting them. Even in the case of 

 Polychaete worms the arguments against interpreting the anatomical 

 arrangements in different genera as illustrating evolutionary sequence 

 in the reverse, order to that believed in by Goodrich seem unconvincing 

 and insufficient to counterbalance the weight of physiological 

 probability. 



In the case of a tube leading from the coelome to the exterior 

 the two ends are almost of necessity mesodermal and ectodermal 

 in their nature respectively. Consequently; the fact that the 

 " nephromixium " has such a twofold origin in ontogeny does not 

 appear to the present writer to constitute evidence of any particular 

 weight that it actually arose in phylogeny by the fusion of two pre- 

 existing independent organs. As regards the proportion derived 

 from the two layers the probability would be that the specially 

 excretory portion was originally ectodermal — excretory products 

 being naturally got rid of by the outer surface — and that the portion 

 specially concerned with the getting rid of coelomic products would 

 be mesodermal — arising as a bulging of the coelomic lining. 



Accepting as a working hypothesis that the nephridial system 

 of tubes with their nephrostomes arose in the manner outlined 

 above, it is important to bear in mind how greatly the system would 

 be influenced in its subsequent evolution by the establishment of 



1 E.g. the separation of the Miillerian duct from the kidney system or the 

 separation of the renal collecting tubes from the Wolffian duct. 



