50 



OLIGOCHAETA 



Fiar. 12. 



§ 3. Development of permanent nephridia. 



The development of the permanent nephridia out of the pronephridia has been 

 followed by Vejdovsky (9) in Shynckelmis and in various species of Lumbricidae. 

 In the former genus the postseptal part of the pronephridium gets thicker and a lobe 

 (woodcut, fig. gl) is formed which lies to the side ; the distal part of the pronephridium 



remains unaltered ; in this condition the nephri- 

 dium recalls that of the Enchytraeidae. The 

 pronephridiostom, originally consisting of one 

 cell with a flagellum, becomes multicellular, 

 yJ ultimately consisting of eight cells ; these become 

 ciliated and the long flagellum vanishes ; but it 

 coexists for some time with the cilia. The 

 vacuole of the pronephridiostom becomes the 

 cavity of the funnel, which comes to open into 

 the coelom and the tube following is gradually 

 excavated. The last part of the permanent 

 nephridium to appear is the epidermic, con- 

 tractile, terminal bladder. Occasionally in the 

 course of development secondary funnels appear, 

 a course of development which recalls the 

 formation of many funnels in some of the 

 ' plectonephric ' worms. In the Lumbricidae 

 the distinction between the pronephridium and 

 the nephridium is not so marked because of 

 the absence of the vacuole and the flagellum in 

 the pronephridiostom ; but in these worms, as 

 in Rhynchelmis, the main part of the permanent 

 organ appears as a lateral outgrowth of the 

 the original straight tube lead- 

 more or less 



unaltered, the alteration concerning the formation of a hollow tube out of it. It has 

 been said by Bergh that there is no epidermic invagination to form the contractile 

 bladder of the nephridium of Lumbricus ; but this is denied by Vejdovsky. There 

 is evidently a greater break between the pronephridia and the permanent nephridia 

 in Rhynchelmis than in Lumbricus. This is emphasised by the existence of the 

 flagellum in the former. 



DEVELOPMENT OF NEPHEIDIA OP 

 RHYNCHELMIS. 

 (After Vejdovsky.) 



The stages are numbered oonseoutively. gl Lote pronephridium 

 forming chief part of Nephridium. 



ing to the exterior remaining 



