Structure iind Development of the Nephridia. 391 



cross section in Piate 25, Figa. 43 aud 44). The cilia of tliese cells 

 become shorter and more numerous (compare Piate 24, Figs. 38, 39; 

 Piate 25, Figs. 42, 46 — 49), wliile at the same time the nuclei become 

 smaller and more rounded, and the nucleoli characteristic of the 

 excretory cells usually disappear. Eventually a well-defined funnel 

 ragion is formed whose cells are sharply distinct froni the excretory 

 cells behiud (Figs. 46 — 49). 



During the earlier stages of the above transformation the inter- 

 segmental septum undergoes changes which lead to the formation 

 of the segmentai blood-vessel. The septum at the beginning of the 

 period in question extends obliquely forward from the ventral region 

 of the stomach (where it is continuous with the sub-intestinal blood- 

 vessel) to the ventral body-wall, forming a thin membrane which 

 extends laterally to a point slightly above the level of the early 

 nephrostome (Piate 25, Fig. 43). In its lateral portion its border 

 becomes continuous with the lip of the early nephrostome, as above 

 described — the anterior face of the septum being directly continuous 

 with the epithelium of the funnel — while the posterior lamella 

 passes directly into the peritoneum cov^ring the nephridium. Figs. 37 

 — 39, Piate 24 show the relations of septum and nephridium at the 

 early period of differentiation of the nephrostome before definite 

 blood-vessels have appeared. 



Bach segmentai blood-vessel appears in the septum (see Piate 25, 

 Fig. 43, in which portions of the originai septum yet remain), origi- 

 nally as a space between the two lamellae of the latter. The vessel 

 begins its formation at the junctiou with the sub-intestinal blood- 

 vessel (Piate 22, Fìg. 9) and gradually extends outward to the body- 

 wall where it joins the lateral longitudinal vessel, also formed about 

 this time. (The relations of the segmentai vessel to the nephridium 

 are in part shown in Piate 25, Figs. 42 — 44). Near its junction 

 with the body-wall the main vessel gives otf a branch (the nephro- 

 stomial vessel) which curves back and passes inward and backward 

 along the dorsal lip of the nephrostome between the funnel-epithe- 

 lium and the peritoneum, to the glandulär portion of the nephridium 

 along which it passes as the main nephridial vessel. The nephro- 

 stomial vessel is thus directly continuous with the segmentai vessel 

 and is formed in essentially the same manner as this latter, repre- 

 senting in reality a space between the posterior septal lamella 

 (represented by the peritoneum) and the anterior lamella at the 

 region where the latter joins the funnel-epithelium with which it is 



Mittheilungen a. d. Zool. station zu Neapel. Bd. 17. 26 



