612 MONTGOMERY- MORPHOLOGY OF THE [April 24, 



plex cells could not have arisen independently in the two groups, 

 rather that their presence in them means homology of the organs 

 concerned. 



It will be seen that my views do not coincide exactly with any 

 of the preceding. I agree entirely with Meyer and Goodrich that 

 the ccelomostome is an organ of origin independent from the 

 nephridium, one that in some cases may connect with the latter. 

 This ccelomostome is equivalent to the genital duct of a lower meta- 

 zoan, as shown by Bergh. I agree also with Lang that the excretory 

 ducts of the protonephridia have maintained themselves in part in 

 the higher Metazoa, and that the longitudinal canals have dis- 

 appeared. But I have tried to show that while sometimes such an 

 excretory duct joins directly with a ccelomostome, forming what I 

 call a coelonephridium, in other cases it joins with retroperitoneal 

 mesentoblastic tissue and the latter may secondarily join with a 

 ccelomostome (metanephridium). In other words, we have to 

 reckon with a retroperitoneal element that frequently forms the 

 greater portion of the nephridium, and this is what Lang and Good- 

 rich have failed to take into account. And I differ from Bergh 

 in concluding that the metanephridium is not in its entirety equiv- 

 alent to a genital duct, but that only a portion of it (the ccelomo- 

 stome) is. Goodrich's mistake, if my interpretation is correct, is in 

 assuming that there are only two elements, ectoblastic tube and 

 peritoneal ccelomostome; he entirely neglects the retroperitoneal 

 tissue, and yet this is just what shows the dyshomology of proto- 

 nephridinm and metanephridium. It is a mistake that has resulted 

 from too exclusive reliance upon phenomena of adult structure 

 with neglect of comparative embryology. And the arguments from 

 histological similarity, intracellular cavity, similarity of solenocytes 

 to flame cells, etc., can have little weight now that we are acquainted 

 with still more striking cases of histological convergence as notably 

 the case of the Malpighian vessels of Insects and those of Arachnids. 

 Goodrich has excellently analyzed the history of the ccelomostome 

 and has thereby greatly clarified our knowledge of nephridia. But 

 he has omitted entirely from his general conclusions the retroperi- 

 toneal element which has come to supplant the protonephridium 



