62 KARM NARAYAN BAHL 



size, we have the later development of integumentary nephridia 

 going on side by side with the appearance and growth of septal 

 nephridia, so that we have an overlapping, so to speak, of the 

 first and second stages. The rudiments of septal nephridia 

 appear in two rows, one on each side of the dorsal vessel. 

 The latter in embryos is single anteriorly but double for the 

 greater part of the posterior portion, and the earliest rudiments 

 of the septal nephridia recognizable in whole preparations lie 

 on both sides of this double dorsal vessel (Text-fig. 4). But 

 while the integumentary nephridia vary in their topographical 

 position from segment to segment, lying close to and away 

 from the nerve-cord alternately, the septal ones lie in two 

 straight rows, nearer the mid-dorsal than the mid-ventral line. 



As their name implies, the septal nephridia develop on the 

 intersegmental septa and, in sections, can be seen to lie just 

 internal to the commissural that connects the dorsal with the 

 subneural blood-vessel. As a septal nephridium develops, the 

 pre-septal portion elongates to form a long narrow tube ending 

 in the funnel, the body of the nephridium, lying in the coelomic 

 cavity behind the septum, develops the limbs and loops of 

 the adult organ, while the terminal duct elongates to run 

 along the septum, parallel and internal to the commissural 

 vessel, to meet its fellow into the supra-intestinal duct mid- 

 dorsally. 



These pairs of nephridia of the second stage differ from the 

 primary integumentary nephridia in that the former develop 

 on the septal wall and have no connexion with the body-wall 

 from the very beginning, and that they develop a septal funnel. 

 Thus we see that septal nephridia are not derived from integu- 

 mentary ones, and have no connexion with them except that, 

 as will be shown later, both types can be traced to the same 

 source. 



When the embryo has developed a pair of septal nephridia 

 in each segment we get to the end of the second stage. At 

 this stage the embryo possesses, in each of its typical segments, 

 two pairs of nephridia, an integumentary pair and a septal one, 

 the former opening to the exterior on the body-wall and lying 



