DEVELOPMENT OF NEPHRIDIA OF PHERETIMA 99 



integumentary pair has reached a fairly advanced stage of 

 development. 



7. 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. 



8. 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 supra- 

 intestinal duct and the lumen of the gut appear soon after 

 the formation of the supra-intestinal ducts. 



9. 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. 



10. 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. e. 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. 



