1908.] 



EXCRETORY ORGANS OF METAZOA. 567 



canals swell into a pair of vesicles ; in the adult these longitudinal 

 canals have disappeared, probably by segmenting into segmental 

 excretory vesicles and nephridiopores. 



Development of the Plectonephridia. — In Megascolidcs each seg- 

 ment has one pair of nephridial anlages, each consisting of a 

 preseptal cell and a postseptal cord ; so far the development is like 

 that of the meganephridia ; then the postseptal cord originates many 

 loops and by a rupture of their connecting bridges the micro- 

 nephridia result; the longitudinal canals connecting the latter arise 

 later and are therefore secondary (Vejdovsky, 1892&). In Mah- 

 benus Bourne (1894) described an essentially similar process: that 

 the funnels degenerate, that the loops form secondary and the 

 latter tertiary branches, until each segment comes to contain about 

 fifty micronephridia. These observations indicate clearly that the 

 plectonephric condition is a modification of the primary macro- 

 nephric by a subdivision of originally single organs. This is the 

 position taken by Vejdovsky, Bourne and Beddard (1892) which 

 is contrary to the hypothesis of Benham (1890, 1891a), Spencer 

 (1889) and Beddard (1891) that the plectonephric condition is 

 primitive and comparable with that of the Plathelminths. Micro- 

 nephridia lack nephrostomes because they are division products of 

 the loops only, and not of the funnels. Therefore A^ejdovsky is 

 probably correct in his conclusion that the micronephridia are 

 homologous with the meganephridia, because both arise from a 

 common anlage, comparable with the embryonic pronephridium 

 of Rhynchelmis. 



In Acanthodrilus deverticula grow out from the intestine, at a 

 region probably anterior to the proctodaium, and join with the 

 plectonephridia of that region of the body; this connection is sec- 

 ondary (Beddard, 1889, 1890, 1892). 



Embryonic Nephridia. — For Rhynchelmis three sets of em- 

 bryonic excretory structures have been found by Vejdovsky 

 (1892a). These are (i) " Schluckzellen," cleavage cells containing 

 canals, which had been previously considered to digest the albumen 

 of the Qgg; (2) larval pronephridia, " Kopfnieren " placed between 

 the germ band and the ectoblast; and (3) embryonic nephridia, 

 which later change into the definitive nephridia. Bergh (1888) 



