70 KARM NARAYAN BAHL 



In this fully-formed integumentary nephridium we have 

 to note the absence of either a coelomic funnel or a solenocyte 

 or ' flame-cell '. During the course of development, when the 

 two ends of the elongated nephridial ridge come close together, 

 one end develops into the terminal duct opening to the exterior, 

 while the other remains blind and does not develop any struc- 

 ture at all. These nephridia develop an intra-cellular canal 

 and cilia like the septal ones ; and, no doubt, the excretion in 

 their case takes place by means of the diffusion of the coelomic 

 fluid through their permeable walls. 



6. Development of the Primary Septal Nephridia. 



When the embryo has acquired a pair of integumentary 

 nephridia in each segment — fully developed in the anterior 

 and in various stages of development in the posterior segments 

 —the second set of nephridia, i.e. the septal, make their appear- 

 ance. These form the second pair of nephridia in the body 

 segments of Pheretima (Text-fig. 1 b). The first fifteen 

 segments of the embryo do not develop this second set of 

 nephridia, which appear only in segments behind the first 

 fifteen. Unlike the primary integumentary nephridia the septal 

 primary nephridia appear on both sides of the dorsal vessel 

 instead of the nerve-cord. They form two rows, one on each 

 side of the dorsal vessel, at a distance of about 160 ju from it 

 in an embryo 9 mm. in length. The alternate or scattered 

 arrangement characteristic of the integumentary nephridia 

 of the first stage does not obtain in these septal ones, which 

 occur in two straight rows. 



These nephridia of the second set have no connexion vnth the 

 body-wall, but appear from a very early stage, as their name 

 implies, as outgrowths on the intersegmental septa. They 

 can then be first recognized in whole mounts as masses of cells 

 on the septum, projecting on its posterior surface. These 

 nephridial masses on the septa can be recognized vdth. cer- 

 tainty at the earliest in embryos, about 8 to 9 mm. in length, 

 which have been opened in the mid-ventral line, their yolk 

 removed and the rest including the endoderm mounted flat. 



