NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA- — SNODGRASS 49 



lip being formed from a neighboring group of septal cells. In 

 Clepsine, Bychowsky (1921) says, the first division of the nephridio- 

 blast is in the plane of the dissepiment, and gives rise to an anterior 

 cell that forms the funnel and the adjacent part of the canal, and a 

 posterior cell that generates the rest of the canal. The latter opens 

 finally to the exterior through an ectodermal invagination. Bergh 



Fig. 20. — Successive early stages in the development of the posterior nephridia 

 of the oligochaete Tubifcx rhniloniiii Lam. (From A. Meyer, 1929.) 



A, a primary nephridioblast developed from a cell of the anterior lamella of 

 a dissepiment. B, proliferation of nephridial cells by transverse division of 

 the nephridioblast. C-F, successive extensions of the nephridial canal within 

 a peritoneal sheath derived from the posterior lamella of the dissepiment ; the 

 canal acquires first an intracellular lumen. G, the canal still more elongate and 

 looped upon itself, attached posteriorly to an epidermal cell, through which the 

 lumen penetrates to the e.xterior, and which later forms the nephridial bladder. 

 H, I, two stages in the final development of the nephrostome in the primary 

 nephridioblast by radial division of the nucleus. 



Dsp, dissepiment ; Luvi, lumen ; NCnl, nephridial canal ; Npbl, nephridioblast ; 

 Mpr, nephropore ; A''^^, nephrostome; PSh, peritoneal sheath. 



claims that there is no ectodermal element in the nephridium of 

 Criodrilns, Rhynchelmis, or Luiiibricus. 



It thus appears to be now well established that the metanephridia 

 of the annelids in general are structures of the nature of coelomo- 

 ducts, formed principally as outgrowths of the posterior walls of the 

 coelomic sacs, but perhaps including a terminal part of variable extent 

 derived from the ectoderm. They are extracoelomic, inasmuch as 

 each nephridial canal is invested in a fold of the coelomic peritoneum. 

 The nephridial organs have always been important subjects in dis- 



