AND ZOOGEOGRAPHY OF IXDIAX OLIGOCH.ETA'. 121 



tube. This larger tube, liowever, is easily vecognized as the peri- 

 toneal covering ; it consists of separate cells, shortly columnar in 

 shape, with the nuclei peripherally situated. 



The spii-al coil (coil) lies more ventrally than the rest of the 

 organ ; it is altogether ventral to the lower end of the oval sac. 

 The spiral consists of about two turns; its apex is below, so that 

 the nephridial tube followed from the nephrostome first descends 

 to the apex, and then ascends as the returning limb of the spiral. 

 On quitting the spiral coil it becomes the twisted loop, a dorsal 

 continuation of the coil, which lies b}^ the side of the lower por- 

 tion of the oval sac. Thus roughly half the vertical extent of the 

 twisted nephridial tube consists of the spiral coil which is below 

 the level of the oval sac, and half of the twisted loop, situated 

 alongside the lower part of the sac ; the total extent of the twisted 

 nephridial tube is about "55 mm. The twisted loop is retained 

 by a mesentery which joins that of the oval sac. 



The twisted loop (I0023) contracts to 12 /t in diameter at its end, 

 and joins the oval sac by the intervention of a spindle-shaped 

 portion (s/j«i.). This part of the organ is 80-100 /a in length and 

 40 /x in greatest thickness ; it is perfectly circular in transverse 

 section, has thick (7^) walls, and a lumen which may be widely 

 patent, or may be almost blocked by faintly pink-staining 

 nucleated cells. 



The oval sac (sac) is much elongated dorso-ventrally, gently 

 carved round the alimentary canal, and somewhat flattened; it is 

 the largest and most conspicuous part of the whole apparatus, 

 being •Q5--73 mm. in length and "17 mm. or more in width (in- 

 cluding its peritoneal covering). It has a very bulky coat of 

 high, clear peritoneal cells (p.), 40 /a, 60 /j. or more in height ; in 

 one case the peritoneal coat formed a layer of cells thicker 

 (•18 mm.) than the whole of the included sac. The nuclei of the 

 peritoneal cells are at various levels. 



The muscular coat (m.) of the sac is remarkably thick — 12jU — 

 and consists principally of circular fibres. Inside the muscular 

 coat is a thin but very obvious connective-tissue coat, staining an 

 intense blue with hfematoxylin, about 1 /a thick. The epithelial 

 lining (ep.) consists of lightly staining low columnar or cubical 

 cells; its height is about 12 fx. — about as thick as the miiscular 

 coat. 



A mesentery, about -32 mm. in width, passes from one border 

 of the sac to the septum ; it consists of a layer of blue-staining 

 connective tissue, continuous with that of the wall of the sac and 

 Avith that of the septum, covered on both faces by peritoneal 

 cells. 



The spindle-shaped portion is similar in its structure and its 

 mesenteric relations to the sac. 



The terminal duct (d.) has a length of -S-'SS mm. in its course 

 from the ventral end of the oval sac to the body -wall, and its 

 diameter is 40 /j-. It comes ofi' just to one side of the ventral end 



