SPIRORBIS GRANULATUS. 407 
between the ribs, about half the size, and found amongst the roots of Fuci and corallines, 
and on stones and shells, is probably a young form of this species. 
Fleming’ (1825) describes the tube of this form as having three ridges and two grooves. 
with two whorls and a distinct central cavity. There are a few irregular transverse wrinkles, 
It is found on old shells, but more frequently on the under-surface of loose stones. He quotes 
Montagu’s description of the animal, and it is clear that he refers to this common form. 
Cuvier’s figure of Spirorbis communis shows no ridges on the shell. 
Alex. Agassiz (1866) gave an interesting account of the development of a form which 
he termed Spirorbis spirillum, Gould, but which appears to have a development similar to 
that observed in Spirorbis granulatus, L. Agassiz’s form was attached mainly to Fucus. 
It differs from Pagenstecher’s species, since the ova are not developed in the operculum. 
The ova, which are reddish brown, are found in strings of two rows on each side of the ali- 
mentary canal in the naked, enlarged part of the body behind the anterior region. After 
deposition they lie between the body and the tube. He found the pelagic condition of the 
larva lasted no longer than eight or ten hours, the period between its escape from the egg 
and its fixation in a tube, for he mentions that “the young Spirordis has attained quite an 
advanced stage of growth when it leaves the tube of the parent and swims freely about (in 
search of a place of attachment) during a night at the outside.” It frequently happened 
that in a single night the sides of the glass vessel were covered with tubes. The “tentacles” 
(branchial filaments) arise on the outside, the new appearing nearer the median line on 
alternate sides, and not in pairs, the corresponding tentacles on each side of the middle line 
being of different lengths. The two eyes are prominent, and can generally be traced in the 
adult. The first tentacle appears on the right, the next on the left, and only later the rudi- 
ment of the odd opercular tentacle covering the right tentacle. Two pairs of the anterior 
bristle-bundles appear, and then a third, whilst the posterior extremity has lengthened, the 
anal ciri have nearly disappeared and two indistinct articulations occur behind the collar. 
Two additional tentacles have arisen between the first pair. Subsequently the slightly 
elongated posterior region has two bristle-bundles, the tentacles bifurcate, and the opercular 
tentacle is more distinctly funnel-shaped. About this stage the young Spirorbis escapes 
from the egg and leads a short free life, and in less than twelve hours has secreted its 
calcareous tube. The further changes are chiefly confined to the anterior extremity. 
S. granulatus, as described by Langerhans? (1880), is a different form. The tube 
is purplish, shining, and with transverse streaks between the two lateral ribs. The collar- 
bristles have a distinct gap, and the serrations are coarser than in the British form, but 
Langerhans’ figures are somewhat diagrammatic. This form appears to have an opercular 
brood-cavity. 
Levinsen (1883) considers that this is not the Sprorbis granulatus of Fabricius nor that 
of Langerhans. He notes that occasionally the three ribs are continued at the aperture 
of the tube into teeth (var. tridentata). 
This is not the Sp. granulatus of Caullery and Mesnil, a species in which the embryos 
are developed in the operculum. 
Miss Bush (1905) observes that the title S. granulatus has been erroneously given -to 
1 «Hdin. Philos. Journ.,’ xii, p. 244. 
2 ¢Zeitschr. f. wiss. Zool.,’? Bd. xxxiv, p. 128, fig. 41. 
