The foot and head of _Nautilus_ are in the adult inextricably grown
together, the eye being the only part belonging primarily to the head
which projects from the all-embracing foot. The fore-foot or front
portion of the foot has the form of a number of lobes carrying
tentacles and completely surrounding the mouth (figs. 2, 3). The
epipodia incline towards each other posteriorly so as to form an
incomplete siphon (fig. 4), a condition which is completed and
rendered permanent in the tubular funnel of Dibranchiata. The
epipodial nature of the funnel is well seen in young embryos, in which
this organ is situated laterally and posteriorly between the mantle
and the foot.
[Illustration: FIG. 5.--View of the postero-ventral surface of a male
Pearly Nautilus, the mantle-skirt (c) being completely reflected so as
to show the inner wall of the sub-pallial chamber, and the four
ctenidia and the foot cut short (drawn from nature by A.G. Bourne).
pe, Penis, being the enlarged termination of the right spermatic duct;
l.sp, aperture of the rudimentary left spermatic duct (pyriform sac
of Owen). Other letters as in fig. 4.]
The lobes of the fore-foot of _Nautilus_ and of the other Cephalopoda
require further description. It has been doubted whether these lobes
were rightly referred (by T.H. Huxley) to the fore-foot, and it has
been maintained by some zoologists (H. Grenadier, H. von Jhering)
that they are truly processes of the head. It appears to be impossible
to doubt that the lobes in question are the fore-portion of the foot,
when their development is examined (see fig. 35), further, when the
fact is considered that they are innervated by the pedal ganglion. The
fore-foot of _Nautilus_ completely surrounds the buccal cone (fig. 6,
e), so as to present an appearance with its expanded tentacles similar
to that of the disk of a sea-anemone (_Actinia_). A.G. Bourne, of
University College, prepared from actual specimens the drawings of
this part in the male and female _Nautilus_ reproduced in fig. 6, and
restored the parts to their natural form when expanded. The drawings
show very strikingly the difference between male and female. In the
females (lower figure), we observe in the centre of the disk the
buccal cone e carrying the beak-like pair of jaws which project from
the finely papillate buccal membrane. Three tentaculiferous lobes of
the fore-foot are in immediate contact with this buccal cone; they are
the right and left (c, c) inner lobes, and the inferior inner lobe
(d)--called inferior because it really lies ventralwards of the mouth.
This inner inferior lobe is clearly a double one, representing a right
and left inner inferior lobe fused into one. A lamellated organ on its
surface, known as Owen's organ, probably olfactory in function (n),
marks the separation of the constituent halves of this double 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
conclusive reasons for dissenting from Owen's view. On the ventral
side an extensive part of the internal surface of the muscular ring is
laminated, forming the so-called "organ of Valenciennes," peculiar to
the female and serving for the attachment of the spermatophores. We
have so far enumerated in the female nautilus ninety tentacles. Four
more remain which have a very peculiar position, and almost lead to
the suggestion that the eye itself is a modified tentacle. These
remaining tentacles are placed one above (before) and one below
(behind) each eye, and bring up the total to ninety-four (fig. 3 v,
v).
[Illustration: FIG. 6.--Male (upper) and female (lower) specimens of
_Nautilus pompilius_ as seen in the expanded condition, the observer
looking down on to the buccal cone e; one-third the natural size
linear. The drawings have been made from actual specimens by A.G.
Bourne, B. Sc., University College, London.
a, The shell.
b, The _outer_ ring-like expansion (annular lobe) of the circumoral
muscular mass of the fore-foot, carrying nineteen tentacles on each
side--posteriorly this is enlarged to form the "hood" (marked v in
fig. 1 and m in figs. 2 and 3). giving off the pair of tentacles
marked g in the present figure.
c, The right and left inner lobes of the fore-foot, each carrying
twelve tentacles in the female, in the male subdivided into p, the
"spadix" or hectocotylus on the left side, and q, the "anti-spadix,"
a group of four tentacles on the right side--it is thus seen that
the subdivided right and left inner lobes of the male correspond to
the undivided right and left inner lobes of the female.
d, The inner inferior lobe of the fore-foot, a bilateral structure
in the female carrying two groups, each of fourteen tentacles,
separated from one another by a lamellated organ n, supposed to be
olfactory in function--in the male the inner inferior lobe of the
fore-foot is very much reduced, and has the form of a paired group
of lamellae (d in the upper figure).
e, The buccal cone, rising from the centre of the three inner lobes,
and fringing the protruded calcareous beaks or jaws with a series of
minute papillae.
f, The tentacles of the outer circumoral lobe or annular lobe of the
fore-foot projecting from their sheaths.
g, The two most posterior tentacles of this series belonging to that
part of the annular lobe which forms the hood (m in figs. 2 and 3).
i, Superior ophthalmic tentacle.
k, Inferior ophthalmic tentacle.
l, Eye.
m, Paired laminated organ on each side of the base of the inner
inferior lobe (d) of the female.
n, Olfactory lamellae upon the inner inferior lobe (in the female).
o, The siphon (mid-foot).
p, The spadix (in the male), the hectocotylized portion of the left
inner lobe of the fore-foot representing four modified tentacles,
eight being left unmodified.
q, The anti-spadix (in the male), being four of the twelve tentacles
of the right inner lobe of the fore-foot isolated from the remaining
eight, and representing on the right side the differentiated spadix
of the left side. The four tentacles of the anti-spadix are set,
three on one base and one on a separate base.]
In the adult male nautilus we find the following important differences
in the tentaculiferous disk as compared with the female (see upper
drawing in fig. 6). The inner inferior lobe is rudimentary, and
carries no tentacles. It is represented by three groups of lamellae
(d), which are not fully exposed in the drawing. The right and left
inner lobes are subdivided each into two portions. The right shows a
larger portion carrying eight tentacles, and smaller detached groups
(q) of four tentacles, of which three have their sheaths united whilst
one stands alone. These four tentacles may be called the
"anti-spadix." The left inner lobe shows a similar larger portion
carrying eight tentacles, and a curious conical body behind it
corresponding to the anti-spadix. This is the "spadix." It carries no
tentacles, but is terminated by imbricated lamellae. These lamellae
appear to represent the four tentacles of the anti-spadix of the right
internal lobe, and are generally regarded as corresponding to that
modification of the sucker-bearing arms of male Dibranchiate
Siphonopods to which the name "hectocotylus" is applied. The spadix is
in fact the hectocotylized portion of the fore-foot of the male
nautilus. The hectocotylized arm or lobe of male Dibranchiata is
connected with the process of copulation, and in the male nautilus the
spadix has probably a similar significance, though it is not possible
to suggest how it acts in this relation. It is important to observe
that the modification of the fore-foot in the male as compared with
the female nautilus is not confined to the existence of the spadix.
The anti-spadix and the reduction of the inner inferior lobe are also
male peculiarities. The external annular lobe in the male does not
differ from that of the female; it carries nineteen tentacles on each
side. The four ophthalmic tentacles are also present. Thus in the male
nautilus we find altogether sixty-two tentacles, the thirty-two
additional tentacles of the female being represented by lamelliform
structures.
_Musculature, Fins and, Cartilaginous Skeleton._--Without entering
into a detailed account of the musculature of _Nautilus_, we may point
out that the great muscular masses of the fore-foot and of the
mid-foot (siphon) are ultimately traceable to a large transverse mass
of muscular tissue, the ends of which are visible through the
integument on the right and left surfaces of the body dorsal of the
free flap of the mantle-skirt (fig. 1, l, l, and fig. 3, k). These
muscular areae have a certain adhesion to the shell, and serve both to
hold the animal in its shell and as the fixed supports for the various
movements of the tentaculiferous lobes and the siphon. They are to be
identified with the ring-like area of adhesion by which the
foot-muscle of the limpet is attached to the shell of that animal. In
the Dibranchs a similar origin of the muscular masses of the fore-foot
and mid-foot from the sides of the shell--modified, as this is, in
position and relations--can be traced.
[Illustration: FIG. 7.--Minute structure of the cartilage of _Loligo_
(from Gegenbaur, after Furbringer)
a, Simple cells.
b, Dividing cells.
c, Canaliculi.
d, An empty cartilage capsule with its pores.
e, Canaliculi in section.]
In _Nautilus_ there are no fin-like expansions of the integument,
whereas such occur in the Decapod Dibranchs along the sides of the
visceral hump (figs. 15, 16). As an exception among Octopoda lateral
fins occur in _Pinnoctopus_ (fig. 38, A), and in _Cirrhoteuthis_ (fig.
38, D).
In _Nautilus_ there is a curious plate-like expansion of integument in
the mid-dorsal region just behind the hood, lying between that
structure and the portion of mantle-skirt which is reflected over the
shell. This is shown in fig. 2, b. If we trace out the margin of this
plate we find that it becomes continuous on each side with the sides
of the funnel. In _Sepia_ and other Decapods (not in Octopods) a
closely similar plate exists in an exactly corresponding position (see
b in figs. 10, 26). In _Sepia_ a cartilaginous development occurs here
immediately below the integument forming the so-called "nuchal plate,"
drawn in fig. 8, D. The morphological significance of this nuchal
lamella, as seen both in _Nautilus_ and in _Sepia_, is not obvious.
Cartilage having the structure shown in fig. 7 occurs in various
regions of the body of Cephalopoda. In all Glossophorous Mollusca the
lingual apparatus is supported by internal skeletal pieces, having the
character of cartilage; but in the Cephalopoda such cartilage has a
wider range.
In _Nautilus_ a large H-shaped piece of cartilage is found, forming
the axis of the funnel (fig. 8, A, B). Its hinder part extends up into
the head and supports the peri-oesophageal nerve-mass (a), whilst its
two anterior rami extend into the tongue-like siphon. In _Sepia_, and
Dibranchs generally, the cartilage takes a different form, as shown in
fig. 8, C. The processes of this cartilage cannot be identified in any
way with those of the capito-pedal cartilage of _Nautilus_. The lower
larger portion of this cartilage in _Sepia_ is called the cephalic
cartilage, and forms a complete ring round the oesophagus; it
completely invests also the ganglionic nerve-collar, so that all the
nerves from the latter have to pass through foramina in the cartilage.
The outer angles of this cartilage spread out on each side so as to
form a cup-like receptacle for the eyes. The two processes springing
right and left from this large cartilage in the median line (fig. 8,
C) are the "pre-orbital cartilages"; in front of these, again, there
is seen a piece like an inverted T, which forms a support to the base
of the "arms" of the fore-foot, and is the "basi-brachial" cartilage.
The Decapod Dibranchs have, further, the "nuchal cartilage" already
mentioned, and in _Sepia_, a thin plate-like "sub-ostracal" or
(so-called) dorsal cartilage, the anterior end of which rests on and
fits into the concave nuchal cartilage. In Octopoda there is no nuchal
cartilage, but two band-like "dorsal cartilages." In Decapods there
are also two cartilaginous sockets on the sides of the
funnel--"siphon-hinge cartilages"--into which fleshy knobs of the
mantle-skirt are loosely fitted. In _Sepia_, along the whole base-line
of each lateral fin of the mantle (fig. 15), is a "basi-pterygial
cartilage." It is worthy of remark that we have, thus developed, in
Dibranch Cephalopods a more complete internal cartilaginous skeleton
than is to be found in some of the lower vertebrates. There are other
instances of cartilaginous endo-skeleton in groups other than the
Vertebrata. Thus in some capito-branchiate Chaetopods cartilage forms
a skeletal support for the gill-plumes, whilst in the Arachnids
(_Mygale_, _Scorpio_) and in _Limulus_ a large internal cartilaginous
plate--the ento-sternite--is developed as a support for a large series
of muscles.
[Illustration: FIG. 8.--Cartilaginous skeleton of Cephalopoda (after
Keferstein.)