Structure and Development of the Nephridia. 067 



more rapidly than tliat of the associateci Peritoneum on its outer 

 side, the former wonld soon exceed the latter in extent aud as a 

 result would tend at first to curve outward and eventually, with the 

 coutinued growth, to become separated from tlie peritoneal layer. 

 A marginal space would thus be formed between the epithelium and 

 the Peritoneum which was originally in contact with it. This space 

 mig'ht later ac(iuire the definite characteristics presented by the 

 tubulär marginal space of the adult ncphrostome. This space indeed, 

 presents many of the characteristics that we should expect to find 

 were such its mode of formation. In cross section, its epithelial 

 wall is much more extensive than its membranous wall; its epithe- 

 lium is continuous with that of the interior of the ncphrostome; its 

 interior is closed on all sides tovvard the body-cavity; and it is lined 

 internally by a thin layer of connective tissue, coutinuous with that 

 which is everywhere present between the peritoneal epithelium and 

 the outer ends of the epithelial cells of the nephridium. Whether 

 or not the conditions of its formation were of the mechanical kind 

 above imagined, there seems little doubt that it represents essentially 

 a large and well defiued lymph space between the peritoneum and 

 the outer surface of the nephrostomial epithelium. 



The body of the nephridium is of a deep brown color; it is 

 spacious and uncoiled, as in all other species of Arenicola^ and tapers 

 slightly towards its posterior termination where it joins the large 

 contractile terminal vesicle. The brown color of these regions, like 

 that of the external integument, is due to the numerous excretory 

 granules contained in their walls. The walls of the terminal vesicle 

 are thinner than those of the tubulär region, and are typically folded 

 in such a manner as to present a somewhat morula-like appearance, 

 as represented in Piate 22, Figs. 1 — 4. This appearance is an 

 iucidental result of the network-like arrangement of the uiuscle-fibres 

 which occupy the position of the interspaces between the elevations 

 of the surface. Each inter-muscular area thus tends, in the con- 

 tracted coiulition of the vesicle, to project beyond the general surface 

 in tlie nianner represented. Both nephridial tube and terminal 

 vesicle are richly supplied with blood-vessels. 



Histology. 



The Ups of the ncphrostome and its interior are lined by a 

 Single layer of columnar ciliated cells which show but slight differ- 

 ences of structure in the different regions. Over the surface of the 



