608 MONTGOMERY— MORPHOLOGY OF THE [April 24, 



ucts, and contain no mesentoblast. I would propose that Hatschek's 

 (1888) term protonephridium be limited to them. 



From such protonephridia the other types of nephridia have 

 probably been derived by the persistence of only the discharge ducts, 

 or portions of them, of the former and by the substitution of mesen- 

 toblastic elements for their other portions. The only elements of the 

 protonephridia that have been retained, it should be repeated, are 

 their distal nephridiopores with more or less of the connectant dis- 

 charge ducts, while the remainder of the protonephridia, all the 

 excretory portion proper, has been replaced by mesentoblastic ele- 

 ments. Accordingly, the two other main kinds of nephridia of 

 which we shall have to speak can be at the most compared only in 

 part with these protonephridia, only their distal nephridioporal ends 

 can be so compared. The more specialized kinds of nephridia have 

 probably originated from the protonephridia, not as further special- 

 izations of them but rather by addition of extraneous elements ; 

 on the whole they are not homologous. 



These more specialized nephridia with mesentoblastic consti- 

 tuents fall into two main groups. 



The first of them consists of types 8 and 9, both of which have 

 in common the union of an ectoblastic duct with the peritoneum but 

 have no retroperitoneal mesentoblast. They are either urogenital, 

 or are homodynamous with genital ducts ( ?also in the Entero- 

 pneusta). Their main difference is that in type 8 the peritoneal 

 invagination is more pronounced as a rule, and that in type 9 the 

 connectant coelom has become exclusively excretory. These differ- 

 ences are not important, and these two types are in general homol- 

 ogous. Until retroperitoneal elements are discovered for them 

 they must be considered distinct from the following; and to them 

 the name ccclonephridhim might be given. 



The second kind of the more specialized nephridia comprises 

 types 10 and 11, both characterized by the union of ectoblast with 

 retroperitoneal mesentoblast. Type ii differs from 10 by the 

 addition of a coelomostome (peritoneal funnel), in the manner made 

 known particularly by the studies of E. Meyer and Goodrich. 

 Their essential peculiarity is the retroperitoneal mesentoblast, not 

 the peritoneal funnel. Hatschek (1888) classed these together with 



