146 NEIWID^. 



into two rings, and its surface is roughened with minute corneous 

 prickles, more or less numei'ous in the various species, and distributed 

 in annular rows or limited patches : the orifice is destitute of tenta- 

 cula, but armed with two powerful jaws curved like a sithe, and 

 serrulated on the inner margin (fig. 1 a,pr). The first segment of 

 the body (fig. cit. s) is often larger than the following ; and from its 

 anterior margin four pairs of tentacular cirri arise {t), which spread 

 out on each side of the head in the form of tapered filaments. The 

 feet are homologous throughout and protuberant, formed of two 

 branches coalescent at their insertion with the trunk ; and each 

 branch is furnished with a spine, or sometimes with two or three 

 spines, and one or two bundles of bristles (fig. 1 b) : the latter are 

 two-jointed, the basilar portion being thickened towards its extre- 

 mity, which is deeply cleft to receive the terminal piece into a sort 

 of socket (fig. 1 c) : this piece is sometimes long, straight and subu- 

 late, at other times short, flattish, and slightly curved. The cirri 

 are always slender and tapered ; we find one at the base of each 

 branch of the foot, and that of the ventral branch is invariably 

 shorter than the one pertaining to the dorsal (fig. 16, c). The 

 branchiae, according to Savigny, are constituent parts of the feet, 

 consisting of three fleshy papillae which occupy their extremity 

 (fig. \b,b). Two of these lobe-like appendages are fixed to the 

 dorsal branch, one under the superior cirrus, and one under the 

 setigerous tubercle ; and the third is situated under the ventral 

 branch, between the setigerous tubercle and inferior cirrus. The 

 form of these papillee, as well as their relative size, often varies on 

 the different parts of the body, but they are found on all the feet, 

 excepting, sometimes, on the second and third pairs, where they are 

 more or less rudimentary. Audouin and Milne-Edwards deny their 

 branchial character, since they are not more vascular than the other 

 lobules of the foot ; but at the base of these organs there is a vascular 

 network, which appears to the naturalists mentioned to be the prin- 

 cipal seat of respiration. The anal segment is always terminated 

 with two styles. 



As of most natural and typical genera in every class of animals 

 and of plants, the species appear to be numerous, and to resemble 

 each other so closely, that it is not, in some instances, easy to decide 

 what should constitute their permanent diagnostics ; or to express, 

 in a few apt words, the minute shades of difterence in certain organs 

 which seem to mark them as distinct species. I am satisfied that, 

 in this genus, the ybrm of the body of specimens preserved in spirits 

 will aiford no specific character ; and that as little reliance can be 

 placed on colour, although this is perhaps more uniformly alike in 

 living individuals. The number of segments is also, as Otho Fabri- 

 cius long ago remarked*, liable to considerable variation, both from 



* " Ceterum numeravi sine respectu magnitudinis segmenta 56, 65, 76, 78, 86 

 in diversis ; igitur de numero nil certi statui posse patet : hunc characterem etiain 

 quam maxime vacillare facile credat, cui mutilatio et rediiitegratio articulorum 

 innotuit ; sub reintegrando enim articulo caudali primum accrescente, reliquis vero 

 successive, a momento conspectus numerus dependet." — Faun. Grcenl.p. 292. " I 



