ON THE BRITISH ANNELIDA. 207 



wards be explained in the Amphitriiee, for directing and impelling upwards 

 along the tube the rejectamenta. It is desirable to remark, at this place, that in 

 the TereMlcc the number of the setiferous feet consitates by far the best, most 

 constant, and most easily determined character for the establishment of the 

 boundaries of species. Between some of the species constituted by Montagu 

 no difference exists but that of age ; the same individual at two periods of its 

 growth is frequently referred to two distinct species. Having no fixed and 

 constant mark of species, his definitions are vague, and quite incapable of 

 verification. The tentacles undergo numerous variations with age, so like- 

 wise do the branchiae ; these organs are therefore valueless for classification. 

 The genus Amphitrita is also tubicolous, and generally fixed ; some species, 

 as A. auricoma, are distinguished however for the power of carrying their 

 tubes about from place to place ; and though tubicolous, are not therefore 

 sedentary. In one species, A. alveolata, the branchiae appear as dorsal ap- 

 pendages of the annuli of the anterior -|ths of the body ; in another these 

 organs occur at two pectiniform vascular processes on either side of the neck. 

 In a third and most beautiful species, which I have recently established under 

 the name of Sabina, the branchiae assimilate themselves in structure and 

 situation to those of the Serpulidans. In this genus, in addition to the hooks 

 and setae, the feet are provided with tactile papillae or cirri. The hooks are 

 disposed on elevated ridges extending round the body from the bases of the 

 feet. First, in A. alveolata, a group of flexible tentacles is gathered round 

 the occiput, which discharge the office of sensation and prehension. On the 

 three first post-occipital rings, branchiae, cutting instruments, and hooks are 

 developed ; each hook-bearing ridge supports at either end a brush of acutely 

 cutting double-edged setae (fig. !27, a). These cutting tools are limited to the 

 three first feet ; they are fitted in the most perfect manner for the uses of 

 " dressing " the materials wherewith the architecture of the tube is raised. By 

 them rough hewn stones are polished, rugged surfaces worn down, and angry 

 projections from the interior of the tube smoothed off. At one extremity 

 these ridges are elongated into branchial processes ; all the feet below these 

 are furnished with setas, which are formed on a totally different plan of 

 structure, and which are evidently intended for a distinct office. Each seta 

 consists of a straight flattened shaft, terminating in an extremely fine point, 

 from either side of which minute sharp teeth point towards the free end 

 of the seta. These setae, from their construction, are obviously designed for 

 pushing ; that is, they constitute the agents by which the worm advances 

 towards the mouth of its tube. Below, and at the base of each setiferous ap- 

 pendage, the three first excepted, a minute papilla is observed, which through- 

 out the anterior |^ths of the body preserves its rudimentary proportions ; 

 near the tail however these papillae augment rapidly in size, and assume the 

 peculiar characters of club-shaped processes, perforated at the distal end by 

 orifices leading into their hollow axes. In these axial channels lies a bunch 

 of singularly formed setae, capable of protrusion to an immense distance 

 beyond the club-ends of the fleshy processes ; each of these setae enlarges at 

 its extremity into an oval sponge-body (fig. 27, 6), from the base of which, in 

 one direction, there proceeds backwards (recurved) an acute seta, and from 

 the end, another needle-like process directed towards the root, projects on 

 the other side. Nothing in nature or art is comparable in perfection of 

 mechanism to these exquisite organs. 



The work of scraping, scouring, planing, mopping and sponging, cecono- 

 mic duties for the discharge of which art has never yet produced a single 

 capable instrument, is performed by these marvellous implements alternately, 

 successively or simultaneously with equal facility and completeness. They 



