ouble lobe.
Each half carries a group of fourteen tentacles. The right and the
left inner lobes (c, c) each carry twelve tentacles. External to these
three lobes the muscular substance of the mouth-embracing foot is
raised into a wide ring, which becomes especially thick and large in
the dorsal region where it is notably modified in form, offering a
concavity into which the coil of the shell is received, and furnishing
a protective roof to the retracted mass of tentacles. This part of the
external annular lobe of the fore-foot is called the "hood" (figs. 2,
3, m). The median antero-posterior line traversing this hood exactly
corresponds to the line of concrescence of the two halves of the
fore-foot, which primitively grew forward one on each side of the
head, and finally fused together along this line in front of the
mouth. The tentacles carried by the great annular lobe are nineteen on
each side, thirty-eight in all. They are called "digital," and are
somewhat larger than the "labial" tentacles carried on the three inner
lobes. The dorsalmost pair of tentacles (marked g in fig. 6) are the
only ones which actually belong to that part of the disk which forms
the great dorsal hood m. The hood is, in fact, to a large extent
formed by the enlarged sheaths of these two tentacles. All the
tentacles of the circumoral disk are set in remarkable tubular
sheaths, into which they can be drawn. The sheaths of some of those
belonging to the external or annular lobe are seen in fig. 3, marked
n. The sheaths are muscular as well as the tentacles, and are simply
tubes from the base of which the solid tentacle grows. The functional
significance of this sheathing arrangement is as obscure as its
morphological origin. With reference to the latter, it appears highly
probable that the tubular sheath represents the cup of a sucker such
as is found on the fore-foot of the Dibranchiata. In any case, it
seems to the writer impossible to doubt that each tentacle, and its
sheath on a lobe of the circumoral disk of Nautilus, corresponds to a
sucker on such a lobe of a Dibranchiate. W. Keferstein follows Sir R.
Owen in strongly opposing this identification, and in regarding such
tentacle as the equivalent of a whole lobe or arm of a Decapod or
Octopod Dibranch. The details of these structures, especially in the
facts concerning the hectocotylus and spadix, afford the most
conclu