Structure and Development of the Nephridia. 365 



otlier hauti, to be excreted by the glandulär epithelium of the 

 nephridia. Further evidence of the excretory funetion of the 

 chloragogen eells will be presented later. At present it is suftì- 

 cient to note that an important part of the excretory funetion is 

 apparently performed in the above manner ; and it seems possible 

 that the increased ciliarv surface provided bv the vascular nephros- 

 tomial processes has proved advautageous in facilitating the removal 

 of solid excretory particles from the coeloniic fluid, and has there- 

 fore been acquired in relation to this funetion (compare Rosa, 

 1903). 



The ventral lip of the nephrostome differs from the dorsal 

 lip in being entire and non-vascular, as in apparently ali other 

 speeies oi Arenicola. It presenta a curious and hitherto undescribed 

 modification in the shape of a large , thin-walled, hollow vesicle 

 (Piate 22, Figs. 2, 4) which occupies its median portion aud is pro- 

 loDged at either end into a tubulär re^ion of narrower calibre form- 

 ing the remainder of the margin of the ventral lip. The entire 

 ventral margin between the anterior and posterior limits of the 

 dorsal lip is thus in reality a thin-walled tube, with a centrai 

 dilatation, constituting the vesicle in question. The greater part of 

 the wall of the vesicle and of its tubulär Prolongation is formed 

 of a ciliated epithelium continuous with that lining the interior of 

 tbe nephrostome (see p. 362). In cross section, the epithelium of 

 the ventral lip is found to curve outward as the margin is reache 

 and to pass without demarcation into the epithelium covering the 

 outer surface of the tubulär region. This epithelium extends around 

 the greater part of the circumference of the vesicle and the marginai 

 tubule; it ceases abruptly, however, at a well defined line an the 

 outer surface (Figs. 2, 6) and the remainder of the circumference 

 is forraed of a thin, translucent membrane which at its junction 

 with the wall of the nephrostome becomes continuous with the 

 peritoneal covering of the nephridium. Near its line of junction 

 with the nephrostomial wall, the membrane may show a feW' blood- 

 vessels derived from the general nephridial network, but towards 

 its junction with the epithelium such vessels are generally wanting. 



The interior of the vesicle is typically completely shut off from 

 direct communication with the body-cavity. Occasioually, however, 

 a few ova may be seen within; these may bave gained access to 

 its interior as oögonia, and bave there undergone further growth 

 and maturation. The thin-walled membranous portion may possibly 



