i9oS.] 



EXCRETORY ORGANS OF METAZOA. (311 



. . . Since in the development of the Annehds the head end of the 

 body precedes and the trunk with its successive segments first later 

 comes to formation, so develops first the first nephridial tree pair, 

 the head kidney adapted to the larval body, whose homology with 

 the water vascular system is not contended even by the opponents 

 of the unit theory, later perhaps a second and possibly still a third 

 similar pair with reduced branching. This most anterior pair of 

 nephridial trees that functions during the earliest larval life, at a 

 time when there is still no secondary body cavity developed in the 

 regions concerned, became in the phylogeny a transitory provisory 

 structure, as can be demonstrated on so many larval organs, while 

 the succeeding nephridial pairs of the trunk segments changed to 

 segmental organs." 



The other main view is that represented by Bergh (1885). Ac- 

 cording to him the larval nephridia of the Ccelomata are homologous 

 with the protonephridia, while the adult nephridia of the Annelids 

 are homologous with the gonadal ducts of the Platodes but not 

 homologous with the protonephridia. Thus he concluded (as 

 Williams did long before) that the segmental organs of Annelids 

 were originally genital ducts and later changed into excretory 

 organs ; while the protonephridia do not communicate with the 

 coelom and never serve as genital ducts. 



Goodrich has recently represented a view that in the main sup- 

 ports Lang's. To him there are " nephridia " proper that never 

 serve as genital ducts ; he considers all of these ectoblastic invagma- 

 tions and essentially homologous. Then, adding materially to the 

 discoveries of Eisig and E. Meyer, amplifying them, he find that 

 upon such a nephridium a coelomostome (peritoneal funnel, genital 

 funnel) may become grafted, giving rise then to a complex " nephro- 

 mixium." To Goodrich all nephridia are essentially homologous, 

 they differ only in being combined or not combined with a coelomo- 

 stome.^^ His argument like Lang's is rather anatomical than em- 

 bryological. Both of these investigators also lay great stress upon 

 the presence in Annelid nephridia of the solenocytes, cells similar to 

 the flame cells of protonephridia ; Goodrich argues that such com- 



^^ In the descriptive part under the caption of Polychseta, Goodrich's 

 ideas are given more in cxtcnso. 



