610 MONTGOMERY— MORPHOLOGY OF THE LApril 24, 



discussion of them has been recently furnished by Lang (1903). 

 There are two main views : ( i ) That the nephridia of all the 

 Metazoa are essentially homologous, and (2) that those of the 

 higher Metazoa are dyshomologous with the protonephridia. 



The first of these has been maintained particularly by Lang 

 (1881, 1884, 1903). To him the starting point is the condition in 

 the Turbellarian Gunda, where there are continuous longitudinal 

 main trunks, and more or less regularly arranged excretory ducts. 

 He holds that such a condition has maintained itself in the case of 

 the plectonephridia of the Hirudinea and Oligochseta, but that it 

 has become modified in other Annelids by the segmentation of the 

 longitudinal trunks. This idea is in a sense a necessary corollary 

 of his view of the close relationship of the Turbellaria and 

 Hirudinea. Besides the similarity in the Turbellaria and the 

 Hirudinea above mentioned, he adduces the following main anatom- 

 ical resemblances. ( i ) Hatschek's contention that in Polygordius 

 the adult nephridia develop as outgrowths from a continuous longi- 

 tudinal canal ; the error of this observation has since been pointed 

 out by Fraipont, Meyer, and Woltereck. (2) The presence of net- 

 like nephridia (plectonephridia) in the Annelids; I have entered 

 into the question of the homologies of these in the descriptive 

 section upon the Oligochseta, and here need only recall that Vejdov- 

 sky's embryological studies have shown that the plectonephric con- 

 dition is secondary, derived from the meganephric. (3) The 

 similarity in histological structure of the two kinds of nephridia. 

 (4) Occurrence of serial provisory larval nephridia in Polychsetes, 

 that closely resemble protonephridia ; that these are homologous 

 with larval protonephridia as well as with the definitive ones, accord- 

 ingly, that the protonephridia are homologous with segmental or- 

 gans. Thus Lang derived (1903) "all the segmental nephridia of 

 the Annelids from the segmental portions of the water vascular 

 system that open externally, on the premise that in the Annelids 

 those canals have not persisted which joined the successive seg- 

 ments of the water vascular system. Such a nephridial segment 

 would have consisted in the ancestors of the Annelids of a pair of 

 water vascular trees with excretory ciliated cells on the terminal 

 ends of the capillary branches, and of a trunk opening outward. 



