THELEPTJS CINCINNATUS. 175 



others follow in succession at the posterior part of each segment. The glandular 

 investment of the region passes above it, and forms a finished edge dorsally. The 

 succeeding processes gradually incline to the lateral region, and, posteriorly, to the 

 ventro-lateral region. The bristles form a vertical fan in each process, narrow at the 

 base and spreading out distally. Moreover, they are arranged in two alternating series, 

 a longer and a shorter, the tips of the latter only projecting beyond the surface. They 

 are shorter than those of Lanice conchilega and less slender, and the wings are more 

 distinct. Each bristle has a pale base, gently dilates into the widest part of the shaft, 

 then forms a nearly cylindrical and slightly narrower region to the wings, after which 

 it tapers to a slender, curved tip. The wings are narrow, both anteriorly and posteriorly 

 — the latter having shorter bristles. As Grube and Marenzeller point out, the bristles 

 do not, as Malmgren observes, go to the posterior end, a considerable number of the 

 terminal segments being devoid of them. Thus in an example from Balta about forty 

 of the posterior segments had no bristles. As the tufts of bristles decrease, the 

 uncinigerous processes become more distinct. In imperfectly preserved specimens the 

 cuticle falls off and the bristles cling to it by their tips, probably from the enlargement 

 caused by the wings, but the edges of the wings may also be concerned. 



The branchiae form two tufts of simple filaments arising from a transverse ridge 

 on each side on the second and third segments, the anterior ridge being the longer, 

 passing also further down the side (ventrally), and with more numerous filaments, 

 which are often prettily waved in a spiral manner when the animal is at rest. They 

 are of a pale straw or deep orange colour with a red streak in the centre from the 

 blood-vessel, which is most distinct immediately after a contractile wave of the body 

 drives the fluid forward. As they arise from two segments they can scarcely be called 

 "one pair" as in the 'Catalogue of Worms' in the British Museum. In a young 

 example, half an inch in length, these organs formed two distinct groups on each side, 

 the first containing two or three filaments of different lengths, the posterior only one. 

 They are enveloped in a transparent structureless cuticle, and the hypoderm has finer 

 cells and granules than the tentacles. Longitudinal and circular muscular fibres are 

 also present. The branchiae seem to vary in the genus, and some of the largest examples 

 of T. cincinnatus from the Arctic Seas have short, thick filaments. As regards origin 

 and structure, the British form corresponds generally with the northern type just 

 mentioned. 



The bristle-tufts range from thirty to forty-one, and in transverse section 

 the bristles are somewhat ovate, sometimes approaching a short or blunt ellipse. The 

 anterior bristles have smooth straight shafts with curved, winged tips which taper to 

 a fine point (Plate CXXVI, fig. 6). The wings are of moderate width and disappear 

 before reaching the tip. The winged tips of the shorter series alone project beyond 

 the setigerous process, and they have a more or less alternate arrangement with the 

 longer series. 



The posterior bristles are much reduced in size and the greater part of the shaft 

 is deeply imbedded in the tissues, the translucent free portion having the shaft somewhat 

 dilated at the commencement of the wings, which are rudimentary, and almost reach 

 the surface of the setigerous process. In the most posterior forms the slender shaft 



