A, Dorsal aspect.
B, Ventral aspect.
a, Arms.
e, Eyes.
fi, Fins.
fu, Funnel.
pa, Mantle.
po, Posterior fossa.
sh, Shell.
te, Tentacular arms.
td, Terminal pallial disk]
[Illustration: FIG. 19.--Digram of shell Belemnite (after Phillips). r.
Horny pen or "proostracum": A, conical cavity or "alveolus," in which
the chambered "phragmacone" (p) is contained: g, "guard," or "rostrum."]
The next modification in the enclosed shell is the addition to it of
secondary deposits of calcareous matter, by the inner surface of the
shell-sac. Successive layers are deposited on the posterior part of the
original shell, whether coiled or straight, and these layers form a
conical mass, which may attain great thickness. A somewhat coiled shell
with such a deposit is seen in _Spirulirostra_ (fig. 17, C) of the
Miocene. In the next stage of modification secondary secretion forms a
long and broad projection of the dorsal lip of the aperture; this is
well developed in the belemnites (fig. 19). Thus in these modified
shells three parts are to be distinguished: the original septate shell,
which has been called the phragmacone; the posterior conical deposit,
called the rostrum or guard; and the anterior somewhat flat projection,
called the proostracum. In the living Dibranchiata other than _Spirula_
the phragmacone and rostrum have become very rudimentary. The shell of
_Sepia_ (fig. 20) consists almost entirely of the proostracum, the
little ventral hollow posteriorly representing the phragmacone, and the
posterior pointed projection, the rostrum. In the _Oigopsida_ the shell
is represented by a proostracum which is no longer calcified by forms a
chitinous plume or gladuius, and a similar rudiment occurs in
_Loliginidac_ (fig. 21) and _Sepiolidae_. Lastly, in the Octopoda the
shell is represented only by small chitinous rudiments to which the
retractor muscles of the head and funnel are attached; these are paired
in _Octopus_, unpaired in other cases as in _Cirrhoteuthis_.
[Illustration: FIG. 20. FIG. 21. FIG. 20.--The calcareous internal shell
of _Sepia officinalis, _the so-called cuttle-bone, a, Lateral expansion;
b, anterior cancellated region; c, laminated region, the laminae
enclosing air.]
[Illustration: FIG. 21.--The horny internal shell or gladius or pen of
_Lohgo_.]
The early appearance of the sac of the mantle in which the shell is
enclosed has led to an erroneous identification of this sac with the
primitive shell-sac or shell-gland of the Molluscan embryo. The first
appearance of the shell-sac in Dibranchiata is shown in figs. 35, 36.
Its formation as an open upgrowth of the centro-dorsal area, and the
fact that it appears and disappears without closing in _Argonauta_ and
_Octopus_, was demonstrated by E. Ray Lankester.
[Illustration: FIG. 22.--The Argonaut in life. (After Lacaze-Duthiers)
Tr. Float: Br.a, anterior arms: Br p, posterior arms: V, the expanded
portion of them, once called the sails; B, the beak; C, the shell; En,
the Funnel.]
In _Argonauta_ (the paper nautilus) the female only possesses a shell,
in which the body is contained; but this is not homologous with the
true shell in other cases; it is a structure _sui generis_ secreted by
the expanded arms of the dorsal pair which are closely applied to it on
either side (fig. 22).
[Illustration: FIG. 23.--Head and circumoral processes of the
fore-foot of _Onychoteuthis_ (from Owen).
a, Neck.
b, Eye.
c, The eight short arms.
d, Long prehensile arms, the clavate extremities of which are
provided with suckers at e, and with a double row of hooks beyond at
f. The temporary conjunction of the arms by means of the suckers
enables them to act in combination.]
[Illustration: FIG. 24.--Male of _Ocythoe catenulata_, Steenstrup
(_Octopus carena_, Ver.), showing the hectocotylized arm. (From
Gegenbaur.)
t^1, t^2, t^3, t^4, The first, second, third and fourth arms or
processes of the fore-foot.
h, The third arm of the right side hectocotylized.
x, The apical sac of the hectocotylized arm.
y, The filament which issues from the sac when development is
complete.
i, The siphon.]
_Head, Foot, Mantle and Mantle-cavity._--If we now compare the
fore-foot of the Dibranchiata with that of _Nautilus_, we find in the
first place a more simple arrangement of its lobes, which are either
four or five pairs of tapering processes (called "arms"), arranged in
a series around the buccal cone, and a substitution of suckers for
tentacles on the surface of these lobes (figs. 15 and 24). The most
dorsally placed pair of arms, corresponding to the two sides of the
hood of _Nautilus_, are in reality the most anterior, and are termed
the first pair. In the Octopoda there are four pairs of these arms
(fig. 38), in the Decapoda five pairs, of which the fourth is greatly
elongated (figs. 15, 16). In _Sepia_, _Sepiola_ and _Rossia_, each of
these long arms is withdrawn into a pouch beside the head, and is only
ejected for the purpose of prehension. In _Loligo_ they are completely
retractile, very slightly so in the majority of the Oigopsida, and in
_Rhynchoteuthis_ they are united to form a beak-like appendage. A
gradual reduction of the tentacular arms can be seen in the Decapoda,
leading to their total absence in Octopoda; thus in _Leachia_,
_Chaunoteuthis_ and others these arms are reduced to mere stumps. In
some _Cheiroteuthidae_ and _Cranchiidae_ the ordinary or sessile arms,
especially the dorsal pairs, are reduced. In the Octopoda they are not
unfrequently connected by a web, and form an efficient swimming-bell,
e.g. in _Cirrhoteuthidae_ and _Amphuretidae_. The suckers are placed
on the adoral surface of the arms, and may be in one, two or four
rows, and very numerous. In place of suckers in some genera, e.g.
_Veranya_, we find on certain arms or parts of the arms horny hooks;
in other cases a hook rises from the centre of each sucker. The hooks
on the long arms of _Onychoteuthis_ are drawn in fig. 23. In various
species of _Cheiroteuthis_ the suckers on the tentacular arms are very
feeble, but the bottom of the cup is covered by a number of
anastomosed epithelial filaments which are used as a fishing-net. The
fore-foot, with its apparatus of suckers and hooks, is in the
Dibranchiata essentially a prehensile apparatus, though the whole
series of arms in the Octopoda serve as swimming organs, and in many
(e.g. the common octopus or poulp) the sucker-bearing surface is used
as a crawling organ.
[Illustration: FIG. 25.--View of the postero-ventral surface of a male
_Sepia_, obtained by cutting longitudinally the firm mantle-skirt and
drawing the divided halves apart. This figure is strictly comparable
with fig. 4. (From Gegenbaur.)
C, The head.
J, The mid-foot or siphon, which has been cut open so as to display
the valve i.
R, The glandular tissue of the left nephridium or renal-sac, which
has been cut open (see fig. 29).
P, P, The lateral fins of the mantle-skirt.
Br, The single pair of branchiae (ctenidia).
a, The anus--immediately below it is the opening of the ink-bag.
c, Cartilaginous socket in the siphon to receive c', the
cartilaginous knob of the mantle-skirt--the two constituting the
"pallial hinge apparatus" characteristic of Decapoda, not found in
Octopoda.
g, The azygos genital papilla and aperture.
'i, Valve of the siphon (possibly the rudimentary hind-foot)
m, Muscular band connected with the fore-foot and mid-foot (siphon)
and identical with the muscular mass k in fig. 3.
r, Renal papillae, carrying the apertures of the nephridia.
v.br, Branchial efferent blood-vessel.
v br', Bulbous enlargements of the branchial blood-vessels (see figs
28, 29).
t, Ink-bag]
In the males of the Dibranchiata one of the arms is more or less
modified in connexion with the reproductive function, and is called
the "hectocotylized arm." This name is derived from the condition
assumed by the arm in those cases in which its modification is carried
out to the greatest extent. These cases are those of the Octopods
_Argonauta argo_ and _Ocythoe catenulata_ (fig. 24). In the males of
these the third arm (on the left side in _Argonauta_, on the right
side in _Ocythoe_) is found before the breeding season to be
represented by a globular sac of integument. This sac bursts, and from
it issues an arm larger than its neighbours, having a small sac at its
extremity in _Ocythoe_ (fig. 24. x), from which subsequently a long
filament issues. Before copulation the male charges this arm with the
spermatophores or packets of spermatozoa removed from its generative
orifice beneath the mantle-skirt, and during coitus the arm becomes
detached and is left adhering to the female by means of its suckers. A
new arm is formed at the cicatrix before the next breeding season. The
female, being much larger than the male, swims away with the detached
arm lodged beneath her mantle-skirt. There, in a way which is not
understood, the fertilization of the eggs is effected. Specimens of
the female _Ocythoë_ with the detached arm adherent were examined by
Cuvier, who mistook the arm for a parasitic worm and gave to it the
name _Hectocotylus_. Accordingly, the correspondingly modified arms of
other Cephalopoda are said to be hectocotylized. J.J.S. Steenstrup has
determined the hectocotylized condition of one or other of the arms in
a number of male Dibranchs as follows:--in all, excepting _Argonauta_
and _Ocythoe_ and _Tremoctopus_, the modification of the arm is
slight, consisting in a small enlargement of part or the whole of the
arm, and the obliteration of some of its suckers; in _Octopus_ and
_Eledone_ the third right arm is hectocotylized; in _Rossia_ and
_Sepiola_ the fourth left arm is hectocotylized along its whole
length, and the fourth right arm also in the middle only; in _Sepia_
the fourth left arm is modified at its base only; in _Sepioteuthis_,
the same at its apex; in _Loligo_, the same also at its apex; in
_Loliolus_, the same along its whole length; in _Ommatostrephes_,
_Onychoteuthis_ and _Loligopsis_ no hectocotylized arm has hitherto
been observed. Thus, speaking generally, it is one or both of the
fourth pair of short arms which are modified in the Decapoda, of the
third pair in the Octopoda. In the pallial cavity are situated one
pair of gills in the Dibranchiata (fig. 25), attached dorsally along
the whole of their afferent borders. On each side of the branchia is a
series of lamellae, least in number in the Octopoda. Each lamella is
transversely folded, and the folds are in turn folded, so that the
respiratory surface is increased. On the somatic wall of the pallial
cavity, between and ventral to the gills, are the following apertures:
the anus and opening of the ink-sac, close together in the median
line; a pair of apertures of the renal sacs, on either side of the
median line; external to the renal orifice, on the left side, the
genital aperture in _Cirrhoteuthidae_ and Myopsida. In other Octopoda,
and in nearly all the Oigopsida among the Decapoda, the genital ducts
are paired in the female, but only the left is developed in the male.
The funnel forms a complete tube in the Dibranchiata, and in the
majority of the Decapoda, as in _Nautilus_, it is provided with an
internal valve projecting from its somatic surface, which allows water
to pass outwards but prevents it passing inwards. The mantle performs
rhythmical respiratory movements of expansion and contraction, the
water entering between funnel and mantle and passing out through the
funnel. In Decapoda the edge of the mantle bears internally on each
side a cartilaginous projection which fits into a corresponding
depression on the external surface of the funnel; this is called the
"resisting apparatus," and serves to make the union of mantle and
funnel firmer during expiration. More powerful expiratory movements
are used for sudden retrograde locomotion through the water.
[Illustration: FIG. 26.--Diagram representing a vertical approximately
median antero-posterior section of _Sepia officinalis_ (from a drawing
by A.G. Bourne). The lettering corresponds with that of fig. 10, with
which this drawing is intended to be compared.
a, Shell (here enclosed by a growth of the mantle).
b, The nuchal plate (here a cartilage).
c, (The reference line should be continued through the black area
representing the shell to the outline below it), the integument
covering the visceral hump.
d, The reflected portion of the mantle-skirt forming the sac which
encloses the shell.
e, The inferior margin of the mantle-skirt (mouth of the pallial
chamber).
f, The pallial chamber.
g, The vertically cut median portion of the siphon.
i, The valve of the siphon.
m, The two upper lobes of the fore-foot.
n, The long prehensile arms of the same.
o, The fifth or lowermost lobe of the fore-foot.
p, The third lobe of the fore-foot.
q, The buccal membrane.
v, The upper beak or jaw. s, The lower beak or jaw.
t, The lingual ribbon.
x, The viscero-pericardial sac.
n.c, The nerve-collar.
cr, The crop.
gizz, The gizzard.
an, The anus.
c.t, The left ctenidium or gill-plume.
vent, Ventricle of the heart.
a.b.v, Afferent branchial vessel.
e.b.v, Efferent branchial vessel.
re, Renal glandular mass.
n.n.a, Left nephridial aperture.
visc.per.apert, Viscero-pericardial aperture (see fig. 29).
br.b, Branchial heart.
app, Appendage of the same.
i.s, Ink-bag.]
_Luminous Organs._--In certain Oigopsida living in deep water, e.g.
_Histioteuthis, Calliteuthis, Histiopsis, Pterygioteuthis_, the
surface of the skin bears photogenous organs directed towards the oral
extremity. Anatomically these consist of a deeper photogenous layer
and a more superficial refracting layer. In some cases, e.g.
_Pterygioteuthis_, they occur even within the mantle-cavity.
_Fins._--In the majority of the Decapoda and in the _Cirrhoteuthidae_,
the mantle is produced into lateral symmetrical expansions which have
the function of fins. They originate at the aboral extremity where
they remain in _Spirula_ (fig. 18). In most other Oigopsida they are
terminal, but more dorsal than ventral, e.g. _Loligopsis_ (fig. 16),
and there may be two on each side, as in _Grimalditeuthis_. In other
cases they extend laterally along a greater length of the body, as in
_Sepia_ (fig. 15). In _Ctenopteryx_ they have a superficial
resemblance to the fins of fishes, consisting of a thin membrane
supported by a series of muscular rods.
_Chromatophores._--These are characteristic of the Dibranchiata,
apparently absent in _Nautilus_. They are originally single cells of
ectodermic origin which sink below the epidermis and become connected
with radiating muscular fibres. The cells are single but multinuclear.
Different cells contain pigments of different colours, yellow, brown,
red or blue. Each cell in life is in constant tremulous movement;
under the influence of nervous excitement the cells are suddenly
expanded or contracted, producing blushes of colour and pallor. By
reflex action of which the afferent stimulus acts upon the eyes as in
fishes, the chromatophores assume a condition which approximates the
colour of the animal to that of surrounding objects. In the Decapoda
there are also reflecting elements which produce iridescent hues.
_Aquiferous Cavities._--In addition to the pockets into which the
tentacular arms of Decapoda are retracted, there are in several
Dibranchiata cavities in the integument which open to the exterior by
special pores but have no communication with the vascular system or
other internal cavities of the body. In _Ocythoe_ there are such pores
on the back of the head and at the base of the funnel; buccal pouches
on the ventral side of the mouth, internal to the arms, occur in some
genera, one in _Loligo_, two in _Sepia_. In some species of _Sepia_
there are pouches in the mantle.
_Alimentary Tube._--The principal differences from _Nautilus_ are the
following:--the mandibles are similar in shape, but are chitinous, not
calcified. In the radula there are three teeth on each side of the
median tooth in each row, except in _Gonatus_, in which there are only
two lateral teeth, and the _Cirrhoteuthidae_, in which the radula has
entirely disappeared. In front of the radula is the so-called tongue,
a fleshy projection corresponding to the sub-radular organ of other
Mollusca.
[Illustration: FIG. 27.--Alimentary canal of _Loligo sagittata_ (from
Gegenbaur). The buccal mass is omitted.
oe, Oesophagus.
v, The stomach opened longitudinally.
x, Probe passed through the pylorus.
c, Commencement of the caecum.
e, Its spiral portion.
i, Intestine.
a, Ink-bag.
b, Its opening into the rectum.]
In most of the Dibranchiata there are two pairs of salivary glands. In
the Decapoda the ducts of the posterior pair unite into a median duct
which opens on the surface of the sub-radular organ. The anterior pair
is but slightly developed except in the Oigopsida. In the Octopoda
there are also two pairs, but the posterior pair, except in
_Cirrhoteuthis_ where they are absent, are large and displaced
backwards, being situated near the oesophageal proventriculus.
Connected with the intestine immediately beyond the pylorus is a
thin-walled caecum, spherical in _Rossia_ and _Leachia_, elongated in
_Loligo_, but usually coiled into a spiral (fig. 27). The hepatic
ducts open into the caecum. The liver is developed as a paired gland,
more or less fused into one in the adult, but the ducts are always
paired. The ducts are covered by a number of glandular follicles
forming what is called the pancreas.
The ink-sac, absent in _Nautilus_, is a rectal caecum developed from
its dorsal wall. It is present in all Dibranchiata except _Octopus
arcticus, O. piscatorum_ and _Cirrhoteuthis_. It consists of a deeper
part or gland proper and a reservoir. It extends to the posterior
extremity of the body in _Sepia_, but in _Octopoda_ is usually
embedded in the surface of the liver. The pigment of the secretion is
melanin, and its function is to produce a dense opacity in the water,
which conceals the animal.
_Vascular System_ (fig. 28).--The ventricle lies in the pericardial
cavity, except in Octopoda where this cavity is much reduced. The
auricles, one pair, are contractile expansions of the efferent
branchial vessels. The heart gives off an anterior or cephalic and a
posterior or abdominal aorta. The vascular system is almost perfect,
arteries and veins being united by capillaries. The principal vein is
a vena cava passing backwards ventrally from the cephalic region and
dividing into two afferent branchial veins, each of which receives a
pallial and an abdominal vein. Each of these afferent branchial
vessels is enclosed in the cavity of a renal organ and is covered
externally by the glandular tissue which forms the excretory part of
the "kidney" (fig. 29). Each afferent vessel is expanded into a
contractile branchial heart, which is provided with a glandular
appendage. The latter corresponds to the glandular masses which are
attached to the afferent branchial veins in _Nautilus_, and to the
pericardial glands of other Molluscs.
[Illustration: FIG. 28.--Circulatory and excretory organs of _Sepia_
(from Gegenbaur, after John Hunter).
br, Branchiae (ctenidia).
c, Ventricle of the heart.
a, Anterior artery (aorta).
a', Posterior artery.
v, The right and left auricles (enlargements of the efferent
branchial veins).
v', Efferent branchial vein on the free face of the gill-plume.
v.c, Vena cava.
vi, vc', Afferent branchial vessels (branches of the vena cava, see
fig. 29).
vc", Abdominal veins.
x, Branchial hearts and appendages.
re, e, Glandular substance of the nephridia developed on the wall of
the great veins on their way to the gills. The arrows indicate the
direction of the blood-current.]
[Illustration: FIG. 29.--Diagram of the nephridial sacs, and the veins
which run through them, in _Sepia officinalis_ (after Vigelius). The
nephridial sacs are supposed to have their upper walls removed.
v.c, Vena cava.
r.d.v.c, Right descending branch of the same.
r.s.v.c, Left descending branch of the same.
v.b.a, Vein from the ink-bag.
v.m, Mesenteric vein.
v.g, Genital vein.
v.a.d, Right abdominal vein.
v.a.s, Left abdominal vein.
v.p.d, Right pallial vein.
v.p.s, Left pallial vein.
c.b, Branchial heart.
x, Appendage of the same.
c.v, Capsule of the branchial heart.
np, External aperture of the right nephridial sac.
y, Reno-pericardial orifice placing the left renal sac or nephridium
in communication with the viscero-pericardial sac, the course of
which below the nephridial sac is indicated by dotted lines.
y', The similar orifice of the right side.
a.r, Glandular renal outgrowths.
w.k, Viscero-pericardial sac (dotted outline).]
_Coelom._--The coelom forms a large sac with a constriction between
the anterior or pericardial division and the posterior or genital
division, and it is produced into lateral diverticula which contain
the branchial hearts; but in the Octopoda the pericardial division is
suppressed and the genital division communicates by long ducts with
sacs containing the appendages of the branchial hearts. The renal sacs
communicate with the pericardium by pores near the external renal
apertures; in the Octopoda the reno-pericardial openings are in the
capsules of the branchial hearts. The genital ducts pass from the
genital coelom to the exterior. They are paired in female Oigopsida
and Octopoda except _Cirrhoteuthidae_, but only the left persists in
the males of all Dibranchiata, and in the female Myopsida.
[Illustration: FIGS. 30, 31.--Nerve-centres of _Octopus_. Figure 30
gives a view from the dorsal aspect, figure 31 one from the ventral
aspect.
buc, The buccal mass.
ped, Pedal ganglion.
opt, Optic ganglion.
cer, Cerebral ganglion.
pl, Pleural ganglion.
visc, Visceral ganglion.
oes, Oesophagus.
f, Foramen in the nerve-mass formed by pedal, pleural and visceral
ganglion-pairs, traversed by a blood-vessel.]
In the oviduct is a glandular enlargement, and in addition to this the
females are provided with the so-called nidamental glands which are
developed on the somatic wall of the pallial cavity, one on each side
of the rectum, except in certain Oigopsida (_Enoploteuthis, Cranchia,
Leachia_) and in the Octopoda, in which these organs are absent. The
latter fact is related to the habit of the majority of the Octopoda of
guarding or "incubating" their eggs, which have little protective
covering. In the other cases the eggs are surrounded by a tough
gelatinous elastic material secreted by the nidamental glands.
[Illustration: FIG. 32.--Lateral view of the nervous centres and
nerves of the right side of _Octopus vulgaris_ (from a drawing by A.G.
Bourne).
bg, Buccal ganglion.
cer, Cerebral ganglion.
ped, Pedal ganglion.
pl, Pleural, and visc., visceral region of the pleuro-visceral
ganglion.
gang. stell, The right stellate ganglion of the mantle connected by
a nerve to the pleural portion.
n.visc, The right visceral nerve.
n.olf, Its (probably) olfactory branches.
n.br, Its branchial branches.]
The vas deferens is at first narrow and convoluted, then dilates into
a vesicula seminalis at the end of which is a glandular diverticulum
called the prostate. By the vesicula and the prostate the
spermatophores are formed. These have a structure similar to those of
_Nautilus_, and in the Octopoda may be as much as 50 mm. in length.
Beyond the prostate the duct opens into a large terminal reservoir
which has been called Needham's sac, and in which the spermatophores
are stored.
_Nervous System and Sense-Organs._--The figures (30, 31, 32)
representing the nerve-centres of _Octopus_ serve to exhibit the
disposition of these parts in the Dibranchiata. The ganglia are more
distinctly swollen than in _Nautilus_. In _Octopus_ an infra-buccal
ganglion-pair are present, corresponding to the buccal ganglion-pair
of Gastropoda. In Decapoda a supra-buccal ganglion-pair connected with
these are also developed. Instead of the numerous radiating pallial
nerves of _Nautilus_, we have in the Dibranchiata on each side (right
and left) a large pleural nerve passing from the pleural portion of
the pleuro-visceral ganglion to the mantle, where it enlarges to form
the stellate ganglion. From each stellate ganglion nerves radiate to
supply the powerful muscles of the mantle-skirt. The two stellate
ganglia are connected, except in _Sepiola_, by a transverse
supra-oesophageal commissure, which represents the pallial cords
united by a commissure above the intestine in Amphineura. The nerves
from the visceral portion of the pleuro-visceral ganglion have the
same course as in _Nautilus_, but no osphradial papilla is present. An
enteric nervous system is richly developed in the Dibranchiata,
connected with the somatic nervous centres through the buccal ganglia,
as in the Arthropoda through the stomato-gastric ganglia, and
anastomozing with deep branches of the visceral nerves of the
viscero-pleural ganglion-pair. It has been especially described by A.
Hancock in _Ommatostrephes_. Upon the stomach it forms a single large
and readily detected gastric ganglion.
[Illustration: FIG. 33.--Horizontal section of the eye of _Sepia_
(Myopsid). (From Gegenbaur, after Hensen.)
KK, Cephalic cartilages (see fig. 8).
C, Cornea (closed).
L, Lens.
ci, Ciliary body.
Ri, Internal layer of the retina.
Re, External layer of the retina.
p, Pigment between these.
o, Optic nerve.
go, Optic ganglion.
k and k', Capsular cartilage.
ik, Cartilage of the iris.
w, White body.
ae, Argentine integument.]
In the Dibranchiate division of the Cephalopoda the greatest
elaboration of the dioptric apparatus of the eye is attained, so that
we have in this class the extremes of the two lines of development of
the Molluscan eye, those two lines being the punctigerous and the
lentigerous. The structure of the Dibranchiate's eye is shown in
section in fig. 14, C, and in fig. 33, and its development in figs. 34
and 37. The open sac which forms the retina of the young Dibranchiate
closes up, and constitutes the posterior chamber of the eye, or
primitive optic vesicle (fig. 37, A, poc). The lens forms as a
structureless growth, secreted by both the internal and external
surfaces of the front wall of the optic vesicle (fig. 37, B, l). The
integument around the primitive optic vesicle which has sunk below the
surface now rises up and forms firstly nearest the axis of the eye the
iridian folds (if in B, fig. 37; ik in fig. 33; Ir in fig. 14), and
then secondly an outer circular fold grows up like a wall and
completely closes over the iridian folds and the axis of the primitive
vesicle (fig. 33, C). This covering is transparent, and is the cornea.
In the oceanic Decapoda the cornea does not completely close, but
leaves a central aperture traversed by the optic axis. These forms are
termed Oigopsidae by C. d'Orbigny, whilst the Decapoda with closed
cornea are termed Myopsidae. In the Octopoda the cornea is closed, and
there is yet another fold thrown over the eye. The skin surrounding
the cornea presents a free circular margin, and can be drawn over the
surface of the cornea by a sphincter muscle. It thus acts as an
adjustable diaphragm, exactly similar in movement to the iris of
Vertebrates. _Sepia_ and allied Decapods have a horizontal lower
eyelid, that is to say, only one-half of the sphincter-like fold of
integument is movable. The statocysts are situated ventrally between
the pedal and visceral ganglia, and are entirely enclosed in the
cranial cartilage. The cavity of each is continued into a small blind
process which is the remnant of the embryonic connexion of the vesicle
with the external surface. The sensory epithelium is at the anterior
end of the vesicle forming a macula acustica, and in the cavity is a
single otolith, partly calcareous and partly organic except in
_Eledone_, in which it is entirely organic. The nerve arises from the
cerebral ganglion on each side and passes through the pedal ganglion.
There is no branchial osphradium in the Dibranchiata corresponding to
that of _Nautilus_, but the olfactory organ or rhinophore near the eye
is present. In _Sepia_ and the majority of the Dibranchiata it is a
simple pit, in some of the Oigopsida it is a projection which may be
stalked.
[Illustration: FIG. 34.--Diagrams of sections showing the early stage
of development of the eye of _Loligo_ when it is, like the permanent
eye of _Nautilus_ and of _Patella_, an open sac. (From Lankester.)
A, First appearance of the eye as a ring-like upgrowth.
B, Ingrowth of the ring-like wall so as to form a sac, the primitive
optic vesicle of _Loligo_.]
_Reproduction and Development._--The modification of one or a pair of
the arms in the male for purposes of copulation has already been
described. In many genera the sexes differ from one another in other
characters also. As a rule the males are more slender or smaller than
the females. The maximum degree of sexual dimorphism occurs in
_Argonauta_ among the Octopods; in this genus the female may be
fifteen times as large as the male, and the peculiar modification of
the dorsal arms for the secretion of the shell occurs in the female
only, no shell being formed in the male. In most cases the females are
much more numerous than the males, but the opposite relation appears
to exist in those Octopoda in which the hectocotylus is autotomous,
for as many as four hectocotyli have been found in the pallial cavity
of a single female. When the hectocotylus is not detached it is
usually inserted into the pallial cavity of the female so as to
deposit the spermatophores in or near the aperture of the oviduct, but
in _Sepia_ and _Loligo_ they are merely deposited on the ventral lobes
of the buccal membrane.
The eggs are laid shortly after copulation. In the Octopoda and in
_Sepia_, _Sepiola_ and _Rossia_, each egg has a separate envelope
continued into a long stalk by which it is attached with several
others in a cluster. In _Argonauta_ the eggs are carried by the female
in the cavity of the shell. In _Loligo_ the eggs are very numerous,
and are enclosed in cylindrical transparent gelatinous strings united
at one end into a cluster.
The Cephalopoda appear to be the only Invertebrates in which the egg
is mesoblastic and telolecithal like that of Vertebrata. This is the
result of the large quantity of the yolk, and the position the latter
assumes in relation to the blastoderm. In all other Mollusca the
segmentation is complete though in some cases very unequal. In the egg
of _Loligo_, which has been chiefly studied (fig. 35), the
protoplasmic pole is at the narrower end of the egg, and segmentation
is restricted to this end, forming a layer of ectoderm cells. From one
part of the periphery of the ectoderm proliferation of cells takes
place and gives rise to a layer of scattered nuclei over the whole
surface of the yolk. The region of proliferation marks the anal side
of the ectoderm, and the layer of nuclei forms the perivitelline
membrane. This process must be regarded as equivalent to the first
stage of invagination, the yolk being surrounded by hypoblast cells or
their nuclei. Later on the same anal edge of the ectoderm forms
another cellular layer, the endoderm proper, which forms a continuous
sheet below the ectoderm.
The mesoderm also originates at the anal side of the ectoderm and
extends in two bands right and left between ectoderm and endoderm.
After the mesoderm is thus established, a little vesicle lying upon
and open to the yolk is formed from the endoderm, and this vesicle
ultimately gives rise to the stomach, the two lobes of the liver and
the intestine. The buccal mass and oesophagus arise from a stomodaeal
invagination, and the anus is formed later from a short proctodaeal
invagination.
The external changes of form are as follows:--The mantle is the middle
of the embryonic area, and in its centre is the shell-gland, which,
however, behaves in a different way from that seen in other Molluscs.
Its borders grow inwards and approach each other to form the
shell-sac. E. Ray Lankester showed that in _Argonauta_ and other
Octopods the shell-sac disappears before it is closed up, but in other
forms except _Spirula_ it closes completely and the shell develops
within it. The lateral and posterior borders of the embryo form the
foot, and these borders grow out into ten or eight lobes which become
the arms, and which at first, as seen in fig. 35 (8), are entirely
posterior to the mouth. Development actually shows the anterior arms
gradually growing round the mouth and uniting in front of it. Between
the mantle and the foot are two ridges which form the funnel, and
their position shows them to be the epipodia. The otocysts and eyes
are formed as invaginations of ectoderm, the former behind the eyes,
at the sides of the funnel. All the nerve-centres, cerebral, visceral,
pedal and optic, are formed as proliferations of the ectoderm. At the
sides of the optic ganglia a pair of ectodermic invaginations are
formed, which in the adult become the white bodies of the eyes,
surrounding the optic ganglion. These are vestiges of lateral cerebral
lobes which degenerate in the course of development.
[Illustration: FIG. 35.--Development of _Loligo_.
1. View of the cleavage of the egg during the first formation of
embryonic cells.
2. Lateral view of the egg at a little later stage. a, Limit to
which the layer of cleavage-cells has spread over the egg; b,
portion of the egg (shaded) as yet uncovered by cleavage-cells; ap,
the auto-plasts; kp, cleavage-pole where first cells were formed.
3. Later stage, the limit (a) now extended so as to leave but little
of the egg-surface (b) unenclosed. The eyes (d), mouth (e) and
mantle-sac (u) have appeared.
4. Later stage, anterior surface, the embryo is becoming nipped off
from the yolk-sac (g).
5. View of an embryo similar to (3) from the cleavage-pole or
centro-dorsal area.