SABELLA PENICILLUS 



223 



in a rounded tip. Ventral groove conspicuous posteriorly as well as in front, not 

 continued on the dorsum anteriorly. Fascicles of bristles dorsal in front, ventral 

 posteriorly, hooks ventral anteriorly, dorsal posteriorly. Anus at the tip, occasionally 

 dorsal. Setigerous processes commence at the buccal segment and continue to the 

 posterior end. Bristles anteriorly either with long narrow wings and slightly curved 

 tips or with shorter and broader tips and wings. 



Uncinigerous ridges from the second segment backward to the posterior end, with 

 dissimilar biserial rows (two) in front; only the avicularian rows (hooks) posteriorly. 



1. Sabella penicillus, Linnaeus, 1766. Plate CXIV fig. 1— body ; Plate CXX, fig. 5— 

 reproduced tail ; Plate CXXVII, figs. 6 — 6 i — bristles and hooks. 



Specific Characters. — Cephalic plate deeply split into two lateral pillars, which carry 

 in each case 35-45 branchial filaments attached to a firm basal ring, the filaments 



*H*J.C. T>C. 



crr\ 



Fig. 150. — Transverse section of the body-wall of the anterior region of Sabella penicillus. The neural canal 



is larger than the nerve-trunk on each side. 



elongate, slender, tapered only at the extremity, which is smooth, the rest having a series 

 of short, delicate pinnae. No ocular points on the branchiae, which show white with 

 purple bands at regular intervals. Collar narrow behind, and passes ventrally to 

 expand into the large reflected lamellae on each side of the median line. Tentacles 

 flattened at the base, canaliculate, sub triangular in section and tapered to a delicate 

 point, about a fourth the length of the branchiae. Body flattened, 12 — 15 inches long, 

 tail rounded and little tapered, segments 150 — 200 or more (600); 8 — 12 thoracic. In 

 an example of 150 segments, breadth 6 mm., branchiae 45 mm. Brownish flesh-colour 

 with the ventral scutes pale. Anterior bristles arranged in a longer and shorter series, 

 the longer having tips with narrow wings, the shorter with broader wings tapered to a 

 fine point. In the second region the bristle- tufts form narrow cones and all of one 

 length, with somewhat short tips and broad wings. Longer and finer bristles of two 

 kinds, the longer without a visible wing, the shorter with longer tips and striated wings, 

 the whole forming a cylindrical column with a lanceolate head enlarged at the base. 

 Avicular hooks anteriorly in a single row, with a bold forward curve of the crown, and a 

 single acute main fang, the free edge being serrated, each being accompanied by a 

 paddle-shaped bristle. Posterior hooks smaller, also in a single row, of similar structure 



