74 KARM NARAYAN BAHL 



septal nephridia at all stages of their dovelopmont (fig. 12). 

 While these two ends of the nephridium — the * centripetal ' 

 end and the ' funnel ' end — are growing and differentiating, the 

 portion of the nephridial mass between the two ends also 

 grows and forms a papilla-like projection behind the septum 

 (fig. 12b). This papilla elongates to form a loop, the two limbs 

 of which come close together ; a bend appears, and the portion 

 distal to the bend forms the rudiment of the short straight lobe 

 of the adult nephridium. This stage of the development of 

 the septal nephridium is represented in fig. 12 c. The two ends 

 of the nephridium are attached to the septum while the body 

 of the nephridium, consisting of a newly-formed straight 

 lobe (s.L), distal to the bend, and a growing region proximal 

 to the bend, between the latter and the attached ends of the 

 nephridium, lies free in the coelomic cavity. As this growing 

 region elongates (fig. 12 d) the two limbs come close together, 

 and, as a result of elongation, twists appear, which grow to 

 form the spirally twisted loop of the adult nephridium (fig. 12 

 and Text-fig. 7). We thus get the main body of the nephridium, 

 consisting of the short straight lobe and the long spirally 

 twisted loop, fully formed. Histological differentiation, 

 along with the formation of intracellular canals with cilia 

 lining them at intervals, completes the development of a 

 nephridium. 



As will be seen by comparing the foregoing account of the 

 development of a septal nephridium with that of an integu- 

 mentary nephridium described in the last section, the successive 

 steps of growth in the two cases are very similar if not identical. 

 The chief difference lies, of course, in the fate of the two ends 

 of the nephridium. In the case of a septal nephridium one end 

 grows out to be pre-septal and is differentiated to form the 

 ' funnel ', the other end forms the terminal duct which runs 

 along the septum, parallel to the commissural vessel, and joins 

 its fellow to form the supra-intestinal duct ; on the other hand, 

 in the integumentary nephridium the ' funnel ' end is bUnd, 

 and the terminal duct opens on the surface of the skin. 



We have now followed the development of a septal nephri- 



