Structure and Development of the Nephridia. 363 



Externally, however, the blood-vessel is not covered by epithelium, 

 but merely by the investing peritoneum of the nephridium (textfig.)- 

 The relations mar, therefore be described as follows: the blood- 

 vessel is bordered by a regulär series of short flattened palmate 

 brauches or folds, which are covered by the eiliated epithelium of 

 the nephrostome and tog-ether with this form the eiliated proeesses 

 of the dorsal lip. 



The manner in which these peculiar relations are established 

 becomes clearer on considering the condition in the later larvai 

 stages. The nephrostome in larvae of from 2 to 10 mm. in length 

 is composed of cubical eiliated cells of small size; the margin of 

 the dorsal lip is entire, and along the outer surface runs the early 

 nci)hrostomial blood-vessel which at this stage is of uniform diameter 

 throughout its length (Piate 25, Figs. 43, 44, 46—49, nst.h.v.). The 

 vessel retains this position throughout life ; in later stages, however, 

 the portion of the nephrostomial epithelium in contact with the 

 blood-vessels seems to undergo a great increase of surface and to 

 become thrown into folds, which eveutually exhibit a typically 

 regulär arrangement and definite structure and constitute the pro- 

 eesses of the dorsal lip. The wall of the blood-vessel preserves 

 its contact with the nephrostomial epithelium and takes part in the 

 folding process, and hence the interior of the proeesses necessarily 

 remains in coutinuity with the lumen of the blood-vessel. The exact 

 manner of the transformation has not been observed, and the above 

 provisionai account is largely suppositional, since the margin of 

 the epithelium remains entire in larvae of 10 — 15 mm. length, the 

 largest I bave as yet succeeded in rearing. In other respects, 

 however, the nephrostome of this stage has essentially the same 

 cliaracteristics as that of the adult, and the formation of a system 

 of folds in the manner above outlined would complete the resem- 

 blance to the adult nephridium. Gamble & Ashworth (1898) bave 



Explanation of Text-figure. 



Somewhat simplified cross section through the nephrostome and adjoining 

 glandulär regìon of the adult nephridium. Section-blauc passes about midway 

 between anterior and posterior borders of the nephrostome (see Piate 22, Fig. 2). 

 The figure shows the relations of eiliated proeesses of dorsal lip [p.d.l], nephro- 

 stomial blood-vessel [nst.v], vesicle of ventral lip [v.v.l), and glandulär region 

 igl.neph.]. The relations of the nephrostomial epithelium to the proeesses and 

 to the vesicle are shown; also those of the membranous wall of the vesicle {m.v.l.) 

 to the epithelium [e.v.l.) and to the peritoneum {p.n.). b.v: blood-vessels; m.s: 

 muscular slip attached to dorsal lip; ex.ep: excretory epithelium. 



