ANATOMY AND CLASSIFICATION OF TEE ARENICOLID^. 513 



a very large size, some of them being over 20 mm. in length 

 (PL 24, fig. 30). In this specimen there is a seventh com- 

 plete nephridium on the right side, but only the normal 

 number (six) on the left side. 



II. The nephridia of the second section of the genus — 

 containing A. ecaudata and A. Grubii — differ from those 

 described above in several constant characters. 



They seldom exceed 7 mm. in length even in the largest 

 specimens of these species. The vascular ciliated processes of 

 the dorsal lip are less flattened than those described in the pre- 

 ceding section ; they are stouter, cylindrical, and digitiform, 

 generally branched distally. The most obvious difference is, 

 however, seen on examining the ventral lip, which in these 

 two species is deeply notched in the centre, the two halves 

 into which it is thus partly subdivided being semicircular 

 in shape. The dark brown secreting portion of the nephri- 

 dium and the rather lighter coloured vesicle are similar 

 to, but not quite as sharply demarcated as those of A. 

 marina. 



A. Grubii. — There are five pairs of nephridia, opening, 

 as in A. Claparedii, on the fifth to ninth cheetigerous 

 annuli (PI. 25, fig. 44). 



In a specimen 35 mm. long the nephridia are 1*3 mm. to 

 1-6 mm. long. The dorsal lip bears about eight short un- 

 branched vascular processes. The nephridia of full-grown 

 specimens are 4 mm. to 7 mm. long. The processes of the 

 dorsal lip are comparatively few in number, there being 

 usually only about 12-15 (figs. 53, 54). They are stout, 

 digitiform or club-shaped outgrowths, often divided into two 

 or occasionally three, distally. There is no sharp distinction 

 between the middle and terminal portions of the nephri- 

 dium. The terminal part is often drawn out into an irregular 

 somewhat foot-shaped sac (fig. 53). There is a reduction of 

 the first nephridium in two of the specimens we have exa- 

 mined. In an adult specimen from Port St. Mary the first 

 right nephridium is represented by a funnel and the first left 

 one is absent, there being only four nephridia on the left 



