DEVELOPMENT OF NEPHRIDIA OF PHBRETIMA 63 



alternately dorsally and ventrally, and the latter opening into 

 the supra-intestinal duct and lying dorsally throughout 

 (Text-fig. 1 b). The vertical ducts leading from the supra- 

 intestinals to the lumen of the gut at each intersegmentum 

 are also formed at the end of this stage. 



In the third stage we have the development of secondary 

 nephridia, integumentary and septal. These begin to appear 

 at a rather late period of development of the embryo, when it 

 is almost fully formed and is about to come out of the cocoon. 

 The circlets of setae are completely formed in all the segments 

 of this age, and while rudiments only can be seen of septal and 

 integumentary nephridia towards the posterior end, we find 

 them in various stages of development anteriorly. 



These secondary nephridia of both types appear indepen- 

 dently of the primary pairs of their segments. The integu- 

 mentary ones appear earlier than the septal, since towards the 

 posterior end we find segments with rudiments of secondary 

 integumentary nephridia but with no traces of secondary 

 septal ones. In both cases these nephridia in their initial 

 stages are lumps of cells having no connexion with the primary 

 nephridia. 



The septal secondary nephridia appear immediately ventral 

 to the primary pair, and develop very much in the same way 

 as the primary pair, their terminal ducts running dorsalwards 

 on the septa and meeting the ducts of the primary nephridia. 

 Some of these nephridia develop a pre-septal funnel like the 

 primary nephridia, but others, as shown in fig. 15 a, develop 

 the funnel in the same segment in which they lie. In this way 

 we get two kinds of nephridia, one kind with pre-septal funnels 

 and the other with funnels in the same segments as the 

 nephridia. Subsequent pairs of nephridia develop similarly, 

 and we get the formation of the septal canals by the union of 

 the terminal ducts of secondary nephridia with those of the 

 primary ones. 



The secondary integumentary nephridia first appear on the 

 body-wall as masses of cells lying beneath the somatic layer of 

 the coelomic epithelium. They do not arise strictly in pairs 



