SPIRORBIS BOREALIS. 409 
7. SPIRORBIS BOREALIS, Daudin, 1800. Plate CX XII, figs. 10 and 10a—operculum and 
branchia; Plate CXXXII, figs. 4--4b—bristles, 4c—tube, 4d—operculum; Plate 
CXXXIII, fig. 11—hook. 
Specific Characters —Collar typical, that is, open dorsally, but continuous ventrally and 
fusing with the lateral (alar) membrane of the first region. Branchiee four! on each side, 
pale or dull yellow with a tinge of green, each filament having a short, slender and slightly 
tapered terminal process, and bearing from sixteen to twenty ciliated pinne, which are not 
quite opposite each other. Operculum saucer-shaped distally (peltate, terminating obliquely, 
Fleming), thin when viewed laterally ; with a short, massive peduncle. Body 4 mm. long, 
widest in front, tapering to a somewhat broad tail having a rounded papilla on each side 
of the anus. Dark brown in front from the hue of the alimentary canal, reddish orange 
posteriorly. Anterior region has three bristled segments. The dorsal bristles of the first 
arise from a nearly cylindrical setigerous process, have less robust shafts than those of Sp. 
granulatus, and these dilate into a flattened process or shoulder distally, with a smooth lateral 
margin, vhilst the distal edge has five rather coarse serrations (Levinsen says four large and 
two small), which increase in size from below upward, the striz on the flattened process 
making them conspicuous. These are followed by the smooth edge of the “bite” as it 
rises to the somewhat “bellied” and finely tapered blade beyond, the edge of which is 
minutely serrated. Amongst these are some slender, simple, tapering bristles. The posterior 
bristles of the third series have broader sickle-shaped tips with long serrations so as to appear 
“feathery.” Anterior hooks borne on somewhat triangular plates narrower at one end 
than the other, the front edge with about forty serrations and the main fang blunt and bifid. 
The upper end of the hook slopes gently from the spinous edge and ends in an indefinite 
and membranous border, whilst from the base of the blunt main fang the shghtly oblique 
outline proceeds a shorter distance than superiorly, and ends in the same indefinite mem- 
branous border. When the main fang is seen from the front, it is broad at the tip, narrower 
at the base and bifid. The posterior hooks are considerably smaller, and the backward curva- 
ture of the upper (anterior) serrated border makes a rounded end instead of an acute angle 
superiorly. The outlines of the base and of the posterior border join the indefinite margin, 
as in the anterior hooks. Hermaphrodite; eggs in the two anterior segments of the abdomen, 
sperms in segments behind. Ova in strings, one, two or three in a row in the tube. Tubes 
abundant on Fuci, sinistral shaped like the shell of a Nautilus, with a deep umbilicus on 
the free surface, the main mass bemg formed by the last coil, which shows transverse 
wrinkles of growth and less evident longitudinal striz, and is slightly bevelled. Aperture 
circular, though a process of the tube projects beyond it on the surface of the 
eaweed. 
SYNONYMS. 
1685. Vermiculus exaiquus albus nautiloides, Lister. Hist., pl. 553, fig. 5 (fide auct.). 
1699-1717. Planorbis minimus, Petiver. Gazophyl. Nat., tab. xxxv, fig. 8. 
1739. Serpula spirorbis, Plancus. Conch., Tab. i, fig. 8. 
1746. Fe ¥ Linneeus. Fauna Suec., p. 535. 
* Fleming says the branchiz are six in number. 
