ON THE NEPHRIDIA OF THE POLYCUJITA. 187 



way the veutral edge is produced downwards into a sort ot 

 hollow groove (figs. 2, 10, 13, 16, v. proL), which becomes 

 directly continuous with the lip of the nephridial funnel, neph.f. 

 This point will be dealt with again later. The ventral pro- 

 longation of the ciliated organ (not ciliated at its lowest ex- 

 tremity) lies very near the epidermis lining the intersegmental 

 groove, being attached to it by a fold of peritoneum (fig. 13). 



The histology of the ciliated organ presents no striking 

 peculiarity. Like that of Nereis,^ it is similar throughout, 

 and shows no trace of glandular structure. No cell walls are 

 visible ; the cytoplasm is even, but stains more deeply towards 

 the ciliated surface (fig. 5). Numerous small round or oval 

 nuclei are situated near the outer surface, chiefly along the base 

 of each ridge. The anterior surface of the organ, which is 

 turned towards the muscle, is lined with flat coelomic epithe- 

 lium (fig. 5, c. ep.). 



A pair of ciliated organs occurs in the anterior region of 

 every segment of the body after the third parapodium. The 

 organs are largest in the median segments, where they attain 

 a length of 2 mm. from the end of one horn to that of the 

 other, and possess from fifty to eighty transverse ridges. 



The Nephridium. — The nephridium of Hesione opens 

 internally into the coelom by means of a simple funnel, pro- 

 vided with long, stiff", curved cilia (fig. 17). The lips are 

 reflected all round the aperture, on one side curving round 

 the dorso-ventral blood-vessel, to which the nephrostome is 

 attached (figs. 2 and 13). A narrow neck of varying length 

 leads from the funnel to a wide and somewhat twisted tube 

 (figs. 2, 16, and 17, neph. t.), which in turn becomes narrower 

 and more convoluted, forming a mass flattened dorso-ventrally, 

 stretching backwards on the floor of the segment (fig. 2). 

 The nephridial tube does not branch in its course. Finally it 

 widens slightly, and runs into the body-wall, through which it 

 opens by a small pore immediately below the base of the 

 parapodium (fig. 2, neph. p.). 



* " On a New Organ in the Ljcoridea, &c.," ' Quart. Journ. Micr. Sci.,' 

 vol. xxxiv, 1893. 



