ANATOMY AND DEVELOPMENT OP PERIPATUS OAPENSIS. 245 



vesicle for the organ, is provided with an extremely thin wall, 

 lined with very large flattened cells. These cells are formed of 

 granular protoplasm, and each of them is provided with a large 

 nucleus, which causes a considerable projection into the lumen 

 of the sac (figs. 20, 29 s). The epithelial wall of the sac is sup- 

 ported by a membrana propria, over which a delicate layer of 

 the peritoneal epithelium is reflected. 



The coiled tube forming the second section of the nephridium 

 varies in length, and by the character of the "epithelium lining 

 it may be divided into four regions. It commences with a region 

 lined by a fairly columnar epithelium with smallish nuclei (fig. 

 28 5 c 1). The boundaries of the cells of this epithelium are 

 usually very indistinct, and the protoplasm contains numerous 

 minute granules, which are usually arranged in such a manner 

 as to give to optical or real sections of the wall of this part of 

 the tube a transversely striated appearance. These granules 

 are very probably minute balls of excretory matter. 



The nuclei of the cells are placed near their free extremities, 

 contrary to what might have been anticipated, and the inner 

 ends of the cells project for very different lengths into the in- 

 terior, so causing the inner boundary of the epithelium of this 

 part of the tube to have a very ragged appearance. This 

 portion of the coiled tube is continuous at its outer end with 

 the thin-walled vesicle. At its inner end it is continuous with 

 region No. 2 of the coiled tube (fig. 28 5 c 2), which is lined by 

 small closely-packed columnar cells. This portion is followed 

 by region No. 3, which has a very characteristic structure (fig. 

 28 5 c 3). The cells lining this part are very large and flat, and 

 contain large disc-shaped nuclei, which are usually provided 

 with large nucleoli, and often exhibit a beautiful reticulum. 

 They may frequently be observed in a state of division. The 

 protoplasm of this region is provided with similar granules to 

 that in the first region, and the boundaries of the cells are usually 

 very indistinct. The fourth region is very short (fijj. 28 5 c 4), 

 and is formed of small columnar cells. It gradually narrows 

 till it opens suddenly into the terminal section {s o t), which 

 ends by opening into the body cavity, and constitutes the most 



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