On the Development of the ‘Enteronephric’ type of Nephridial system found in Indian Earthworms of the genus Pheretima.’
Authors: Bahl Kami Narayan 
Published: April 2021

The three kinds of nephridia—integumentary, septal, and pharyngeal—appear at successive stages of development of the embryo ; the integumentary preceding the septal and pharyngeal, both of which develop simultaneously. All the three kinds can be traced back to the original row of nephridial cells of ectodermal origin. Thus all the different nephridia are ultimately derived from one common origin. The primary pair of integumentary nephridia are the first to appear from a ‘retro-peritoneal’ group of cells. The rudiments lack the ‘funnel-cell’, and consequently a ‘coelomic’ funnel is never developed in these nephridia. They open to the exterior on the body-wall. These primary integumentary nephridia do not appear in the same position in successive segments of the embryo, but are irregularly distributed all over the body-wall. The septal primary nephridia can be traced back to a group of nephridial cells, including the ‘funnel-cell’, which make their way into each septum between its two adjoining peritoneal lamellae. The primary septal nephridia have always a well-developed pre-septal funnel, and appear along a straight line on both sides of the dorsal vessel. They appear after the primary integumentary pair has reached a fairly advanced stage of development. The secondary nephridia of both the integumentary and septal types are not budded off from the primary nephridia, but the rudiments of all have a common origin and separate early. They resemble the primaries in every respect, except that in the case of the septal secondaries the funnel is either pre-septal or post-septal. The terminal ducts of the primary septal nephridia form the dorsal portions of the septal excretory canals on the septa, and the canals of both sides form the supra-intestinal duct on meeting the mid-dorsal line above the gut. The segmental ductules establishing a communication between the supraintestinal duct and the lumen of the gut appear soon after the formation of the supra-intestinal ducts. The primary pharyngeal nephridia of the fourth, fifth, and sixth segments develop from a ‘retro-peritoneal’ group of cells like the integumentary ones, and have long ducts reaching the wall of the pharynx. Secondary nephridia are formed as successive buds on the ducts, anterior to the primary nephridia. The possible phylogenetic stages in the evolution of the ‘enteronephric’ type of nephridia are as follows : (1) the severance of the connexion between the septal funnel and the body of the nephridium ; (2) migration of the severed portion, i. o. the ‘funnel-cell’, together with some other nephridial cells from a ventral to a lateral position in the embryo ; (3) the growth of this severed portion into a septal nephridium and the acquisition by the latter of an opening into the gut; (4) the elongation of the terminal ducts of all septal nephridia towards the mid-dorsal line (induced by the course of commissural vessels) and the formation of continuous supra-intestinal ducts. It is problematic whether the severance of the connexion between the funnel and the body of the nephridium took place before or after the connexion of the nephridium with the gut.