242 POTAMILLA TORELLL 



wings. The others in this region have two groups — an upper with longer shafts, more 

 tapered and slightly curved tips with narrow wings (Plate CXXVIII, fig. 3), and a dense 

 lower group of spatulate forms (Plate CXXVIII, fig. 3 a) with a process at the tip. 

 The bristles of the middle region form the usual bristle-pillar of rather short bristles 

 with striated shafts, comparatively broad striated wings and very finely tapered tips, two 

 series, a longer and a shorter, being conspicuous. The shaft has a distinct curvature 

 at the junction with the tip. The posterior bristles are fewer in number, and have wings 

 distinctly striated and very attenuate tips (Plate CXXVIII, fig. 3:6). In glancing at 

 small preserved specimens, it is found that most have the posterior bristles directed 

 forward, and in several the anterior have the same direction. In the larger 'forms the 

 anterior bristles are often directed upward, outward and slightly backward. The anterior 

 hooks, which occur on all the anterior segments except the first, are avicular (Plate 

 CXXVIII, fig. 3 c), with a sharp main fang and above it a series of minute teeth on the 

 crown (Langerhans shows about twenty-four) and a moderately long base. Strige pass 

 from the neck to the base, after curving round the prow. These hooks are accompanied 

 by the short bristles with the spatulate tips. The posterior hooks differ only in their 

 smaller size and the brevity of the base. In comparing the larger with the smaller forms 

 from Plymouth, the essential characters of the bristles and hooks are as well shown by the 

 smaller as the larger. The tube is composed of a tough internal lining coated with fine 

 sand-grains, the whole being firm and resistant — especially in the Canadian examples. 

 In the spirit-preparations it often happens that the annelid can only be removed from 

 its tube by the rupture of the tissues. Lo Bianco mentions the occurrence of a colony 

 of Balani and of Gellepora on the tube, and that occasionally the annelid lives in a sponge 

 (Hircinia) at considerable depth off Naples. 



Reproduction.— Specimens from Port Erin in the second week of September had 

 large and apparently nearly ripe ova. The great numbers of this species and of all sizes 

 on certain sites is noteworthy. 



The Sabella neglecta of Sars 1 appears to be a closely allied if not identical form. It 

 is devoid of tentacles and the tips of the branchial filaments are short. 



Arnold Watson 2 (1906) gives an account of a rock-boring Potamilla, probably P. 

 Torelli, at Tenby, which reproduced both anterior and posterior ends, and which, he states, 

 formed the anterior region by the addition of one segment only, the adjoining " abdominal " 

 region having its bristles and hooks reversed to suit the normal condition of the parts. 



Treadwell 3 mentions the occurrence of paired eyes just behind the tip of the 

 branchias in an example from Honolulu. Such has not been observed in British waters. 



Mesnil and Caullery (1911) give an interesting account of remarkable Protozoan 

 parasites (Haplosporidium potamilla and levine) which they found in the tissues of this 

 species, and which they could not readily compare with anything known. The parasites 

 displaced the alimentary canal in the infected segments. The memoir is illustrated by 

 excellent text-figures and plates. The same authors 4 have recently (1920) recorded the 



1 ' Keise Lofot. og Finm./ p. 83, and ' Christ. Selsk. Forhandl./ p. 31 (sep. copy). 



2 f Proceed. Koy. Soc./ vol. lxxvii, p. 332, 4 text-figs. 



3 'U.S. Comm. F. & ¥./ 1906, p. 1178. 



4 'Comp. Rend. Paris/ October, 1920, p. 683. 



