ARENICOLA MABINA. 65 



fourth annuli represent a segment which has lost its seta3. In the peristomium are the 

 statocysts which communicate with the exterior by a narrow tube opening at the point 

 where the " metastomial groove crosses the interannular groove " (Ashworth). Grube 

 and Stannius thought them appendages of the (esophageal connectives. Siebold, again, 

 first pointed them out as sense-organs, and he was followed by De Quatrefages, Frey, 

 and Leuckart. 



The Body may be divided into three regions (after Ashworth) : (1) an anterior 

 bearing feet, but no gills ; (2) a middle region bearing both feet and gills ; and (3) a 

 posterior which has neither gills nor bristles. One of the annuli of each segment is 

 larger and generally more prominent than the others and bears the feet, dorsally the 

 conical process carrying the bristles, and dorso-ventrally the rows of hooks on a muscular 

 ridge. The first few rows are small, but by-ancl-by they almost meet in the mid-ventral 

 line. 



There are thirteen branchiferous segments, and in front of the first gill are six 

 setigerous segments. Each has four rings, except between first and second gill, where only 

 two rings occur, and between second and third, where there are two rings (laminarian 

 variety) or three, as in the littoral variety. The second groove behind each setigerous 

 annulus marks the posterior limit of that segment (where the internal septse are fixed). 

 Ashworth holds that these gills are not developments of dorsal cirri. 



Between the first setigerous annulus and the prostomium is a region of four rings, 

 each of the first three of which may again be subdivided into two. This region is 

 composed of peristomium and a body-segment, the setas of which (a single one, Benham) 

 are minute, and disappear early. The anterior region is the peristomium, and bears the 

 otocysts. The first setigerous segment in the adult is thus the third body-segment. 

 Ashworth is of opinion that the " giant nerve-cells " bear out the foregoing inter- 

 pretation. 



The presence of a vestigial seta in the post-larval forms of A. marina and A.ecaudata 

 indicates a segment between the peristomium and the first adult setigerous segment (Gamble 

 and Ashworth). This appears to be confirmed by the presence of a single " giant-cell " 

 at the meeting-point of the connectives in this species, in A. ecaudata, and in A. cristata. 



The gills have 9 — 11 stems, each with 3 — 5 pairs of short lateral branches (Plate 

 LXXXVII, fig. 6) in the littoral variety. In the laminarian variety (16 in. long) the 

 gill is pinnate, consisting of about twelve stems, united by membrane at their bases, and 

 having ten or more branches on each side of the axis. It differs from the littoral variety 

 also in the subdivision of the interval between the second and third setigerous annuli. 

 In the laminarian it is divided into two rings, in the littoral into three. There is no 

 other structural difference. 



Gamble and Ashworth note two types of gill in this form, viz. the dendritic and the 

 large feathered or pinnate. The branches are only connected at their bases by a kind of 

 web. The gills contract from behind forward, and thus assist in the circulation. 



The mouth is a crescentic transverse slit, on the antero-ventral aspect of the peris- 

 tomium, through which the proboscis is protruded. This organ is pinkish in young, 

 darkly pigmented in old examples. The proboscis in extrusion does not appear to differ 

 materially from that of the other British species. Its base has a series of larger papillge, 



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