94 TEREBELLIDS. 



siderably stronger than in Terebella and pull the lateral regions inward, so that the area 

 of each ventral longitudinal muscle forms a rounded and prominent ridge on each side. 

 The dorsal longitudinal and the ventral longitudinal muscles are about equally developed 

 though from the firm boundary of the ventro-lateral spaces the latter are occasionally 

 thicker. Two strong muscular strands pass from the lower edge of the alimentary canal 

 to the circular coat in the mid-ventral line. A noteworthy feature is the attachment of 

 the oblique muscles (om.) above the feet, instead of the fibres leaving the foot at its 

 lower border beneath the bristles. The nerve-cords lie outside the circular muscular 

 coat as two large and prominent areas, the ventral groove being dimpled in transverse 

 section. The hypoderm covering the prominent ventral ridges is remarkably thick till 

 it is well within the deep median furrow, where it abruptly thins off at the sides of the 

 nerve-cords, which appear to be covered only by a thin layer which forms a kind of 

 modified cuticular covering. The cords are thus in close relation to the surrounding 

 medium. The size of the trunks is of interest in connection with the brilliant phos- 

 phorescence of the species. The segmental organ seems to open below the upper attach- 

 ment of the oblique muscle. The alimentary canal is typical. 



In Terebellides stroemi, Sars, the hypoderm is somewhat firm and granular. The 

 circular muscular coat is thin, though in the dorso-lateral regions its fibres &re increased. 

 The dorsal longitudinal muscles are much extended, and a lateral muscle is cut off by a 

 powerful band which passes from the foot to the outer edge of the ventral longitudinal, 

 the body thus having a rounded ridge on each side ventrally. The nerve-cords are small, 

 and lie within the hypoderm and strong circular muscular coat between the somewhat 

 flattened ventral longitudinal muscles. The slender oblique are attached on each side of 

 the trunks, unless it be held that the strong band cutting off the lateral longitudinal 

 corresponds to these muscles. 



The nomenclature of the Terebellids in former years seems to have been peculiar. 

 Thus, Dujardin's 1 Sabellina brachycera is a form with seven tentacles and eyes on the 

 upper fold as in a young Terebellid. On the other hand, his Sabellina tenuis* in the same 

 paper with ten filiform tentacles and four ocular points behind them and two on the tail 

 resembles Ehrenberg's Amphicora. 



Hessle (1917) does not think the distinctions into fore, middle and hind brain 

 according to Nilsson's view hold in the Terebellimorpha, for the cephalic ganglia are 

 reduced to a nervous band over the pharynx and the distinctive fusions between the parts 

 are absent. In most of the Terebellids the nuchal organ is in abeyance, but in Thelepus 

 and allied genera it is a small ciliated groove on each side. 



Nilsson thought the tentacles derived their nervous supply from the posterior region 

 of the fore brain, but Hessle doubts this interpretation. 



The segmental organs in Terebellids occur as looped organs in the anterior region 

 with internal and external openings, and ranging from four to sixteen or more in 

 number. In Lanice conchilega, however, Cunningham 3 describes four functional organs 

 in segments 6—9, besides traces of three others in front of these, and fused membranous 



1 ' Ami. Sc. nat./ 2 e ser., t. xi, p, 292, pi. vii, figs. 6—8. 



2 Ibid., p. 291, 1839. 



3 Quart. Journ. Micr. Sci., N.s., vol. xxviii, p. 250, pi. xviii, figs. 11 — 13. 



