414 SPIRORBIS BOREALIS. 
of which are bent backward and provided with wings (Plate CX XXII, fig. 4a).1_ The four 
anterior bristles of the third series agree with the foregoing in the finely tapered, winged tips, 
but the posterior five have broader sickle-shaped tips, with a smooth wing at the base, and 
the forward curve of the sickle-like blade shows long serrations so that they are pectinate, 
the tip being finely attenuate (Plate CX XXII, fig. 45). The serrations of these bristles 
are nearly at right angles to the long axis of the tip, and thus show a marked 
differentiation. 
The anterior hooks occur in two rows in the segments bearing the second and third 
bristles, about twenty-three to fifty-five (De St. Joseph) in each. The hook forms a some- 
what triangular, transparent plate (Plate CX XXIII, fig. 11), the anterior edge armed with 
about forty fine teeth. The main fang is blunt and projects little beyond the tip of the 
organ. De St. Joseph considers that it is larger than the others, but Miss Bush again observes 
that she has not seen this in any form. ‘Transverse lines pass across the body of the hook 
from each spike or serration. When viewed from the front the blunt main fang is bifid. 
The posterior hooks are similar, but smaller, and the number of teeth in the row is 
considerably less (six to eighteen). . 
The sinistral tube is shaped like the shell of a Nautilus with a deep umbilicus on the 
free surface, and its main bulk is composed of the last coil which often hides the earlier 
coils, though in some these are distinct in the centre. The surface of the last coil is slightly 
bevelled and marked by transverse lines of growth. The aperture is circular, though a 
process of the tube projects beyond it on the surface of the seaweed. The tubes sometimes 
occur in rows, and on both surfaces of the Fucus. When two are near each other, and a 
third settles between them, the latter generally has an oblique position as it grows, so that 
the aperture may point more or less upward. Five coils are visible in some, the central 
being small, the outer much larger. De St. Joseph met with Folliculina ampulla in 
the interior of a tube, and the ova of Automolos unipunctatus, Graft (Monocilis 
unipunctatus, Cirst.); and Turbellarians are not uncommon in tubes from St. Andrews 
and Lochmaddy. 
Reproduction.—Spirorbis borealis is hermaphrodite, the eggs being developed in the first 
two segments of the posterior region, and the sperms in the segments behind. The eggs 
are large and reddish brown, with a diameter of 0°18 mm., and form a simple, double or 
triple necklace in a transparent investment fixed to the tube. De St. Joseph observes that 
the embryos develop much as described by Willemoes-Suhm, and as already mentioned 
under Sp. granulatus. 
Gotte (1881) describes the development of S. nautiloides, which has a bilaterally sym- 
metrical gastrula. 
Cunningham and Ramage found it breeding in the beginning of June, the ova forming 
a cylindrical cord of “ two or more” linear series in the tube. The embryos were well 
advanced before hatching. 
Willemoes-Suhm? (as S. nautiloides) found the larve at Kiel im June, the adult being 
hermaphrodite, with the eggs in front and the sperms in the posterior segments. The ova 
are deposited in the tube. He points out that Pagenstecher describes the development of 
" Loye figures the simple winged bristle with a serrated edge, Taf. xvi, fig. 11. 
* “Zeitschr. f. wiss. Zool.,’ Bd. xxi, p. 394, Taf. xxxi, figs. 9, 10 and 13. 
