igoS ] 



EXCRETORY ORGANS OF METAZOA. 609 



the preceding as metanephridia, and diagnosed them by the presence 

 of a coelomostome ; but the difference with regard to the retroperi- 

 toneal element seems to me so important that these should be held 

 distinct from the preceding, and in that case it would be well to 

 limit the term metanephridium to types lo and ii. 



The three main kinds of nephridia that these considerations lead 

 us to distinguish may be briefly compared as follows : Protonephrid- 

 ium (types 3, 4, 7), wholly ectoblastic or mesectoblastic (possibly 

 in some cases both ectoblastic and mesectoblastic) ; ccelonephridium 

 (types 8, 9), distal ectoblastic portion joining directly with a coelomo- 

 stome; metanephridhun (types 10, 11), distal ectoblastic portion 

 joining with retroperitoneal mesentoblast, and the latter connecting 

 or not connecting with a coelomostome. Only the second and third 

 of these ever serve as genital ducts or are homodynamous with them. 

 The metanephridium is the most complex because it may consist of 

 as many as three elements, and it contains the smallest amount of 

 the ectoblastic constituent. 



The protonephridium in the course of transmutation and division 

 of labor has not become entirely replaced, but it has rather become 

 reduced in amount by the substitution of other elements for certain 

 of its parts. And there have been two paths in this process. By 

 the one, a relatively larger portion of the protonephridium has per- 

 sisted and a coelomostome has become directly connected with it, 

 exemplified by the ccelonephridium. By the other a relatively 

 smaller portion of it has maintained itself, to this has been added 

 a secretory tube of retroperitoneal mesentoblastic tissue, and to the 

 latter in some cases a coelomostome, as illustrated by the meta- 

 nephridium. The coelomostome is homologically a genital funnel, 

 as demonstrated by ]\Ieyer and Goodrich, comparable with a genital 

 duct of, e. g., a Nemertean. But what the retroperitoneal mesento- 

 blastic element was originally, before it attached itself to a proto- 

 nephridium, we are unable to decide ; it may have originated from 

 the outer layer, that outside of the peritoneum, of a primitive 

 gonadal pouch. 



We have now to see how these conclusions relate themselves 

 to the views of other students. It will not be necessary to attempt 

 a full historical review of the various opinions because a o-Qod 



