Structure and Development of the Nephridia. 371 



vasculav supply of the young' iiephridium is much less developed 

 thau in the adult; aud the cells of tlie terminal vesicle are flatter 

 and eoutain fewer granules, wliile the muscular layer of its wall is 

 larg-ely incomplete (Piate 25, Figs. 45, 55). 



The numbei- of granules in the excretory cells of larvai stages 

 is however subject to great Variation, as will be seen by a reference 

 to Figs. 46 — 54 which represeut cross sections from the first nephri- 

 dium of a single specimen of somewhat smaller size than the 

 one from which Fig. 7 was taken. In this specimen the nephridial 

 cells contain numerous and characteristic excretory granules; in other 

 respeets, however, they are esseutially similar to those of the first 

 specimen. The projection of the individuai cells into the lumen is 

 present bere as in later stages, though it is less market! . 



Later the histology of the larvai nephridia will be dealt with in 

 more detail. 



5. Development of the Nephridia. 



The nephridia develop in dose relation to the other segmentai 

 structures of the body, especially the septa with which they are 

 from the first in direct continuity. The early somites, as in other 

 Aunelids, raake their appearance as immediate products of the acti- 

 vity of a growing zone, comparable with the teloblastic region of leech 

 or oligochaete embryos. It will be uecessary therefore, in order to 

 make clear the earliest relations of the nephridia, to consider in 

 greater detail the characteristics of this zone and of the region of 

 active differentiation immediately anterior to it. 



(a) Growing Zone. 

 The appearance of the growing zone in longitudinal section is 

 represented for early larvae in Plates 22 and 23, Figs. 7, 8, 9, 10; for 

 somewhat later stages in Plates 23 and 24, Figs. 30 — 32 ; and for the 

 stage of formation of the achaetous body-region in Piate 24, Fig. 33. 

 Its appearance in cross section may be seen from Figs. 14, 21 and 

 24, Piate 23. Its characteristics remain almost Constant for the whole 

 period of its activity; after the formation of 56 — 58 somites it dis- 

 appears and the formation of somites ceases. Throughoiit the whole 

 period of somite-formation the zone retains a Constant position a 

 short distance in front of the posterior extremity of the body, imme- 

 diately anterior to the post-mesodermal (pygidial) or form er para- 

 trochal region. Posteriorly where it adjoius the pygidial region its 



