Chapter VII: Part 7

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.

A, Capito-pedal cartilage of _Nautilus pompilius._

a points to the ridge which supports the pedal portion of the
nerve-centre.

B, Lateral view of the same--the large anterior processes are sunk
in the muscular substance of the siphon.

C, Cephalic cartilages of _Sepia officinalis_.

D, Nuchal cartilage of _Sepia officinalis_.]

_Alimentary Tract._--The buccal cone of _Nautilus_ is terminated by a
villous margin (buccal membrane), surrounding the pair of beak-like
jaws, of which the ventral projects over the dorsal. These are very
strong and dense in _Nautilus_, being calcified. Fossilized beaks of
Tetrabranchiata are known under the name of rhyncholites. In Dibranchs
the beaks are horny, but similar in shape to those of _Nautilus_. They
resemble in general those of a parrot, the lower beak being the larger
and overlapping the upper or dorsal beak. The lingual ribbon and
odontophoral apparatus have the structure which is typical for
Glossophorous Mollusca. In fig. 9, A is represented a single row of
teeth from the lingual ribbon of _Nautilus_, and in fig. 9, B, C, of
other Cephalopoda.

In _Nautilus_ a long and wide crop or dilated oesophagus (fig. 10, cr)
passes from the muscular buccal mass, and at the apex of the visceral
hump passes into a highly muscular stomach, resembling the gizzard of
a bird (fig 10, gizz). A nearly straight intestine passes from the
muscular stomach to the anus, near which it develops a small caecum.
In other Cephalopods the oesophagus is usually narrower and the
muscular stomach more capacious, whilst a very important feature in
the alimentary tract is formed by the caecum. In all but _Nautilus_
the caecum lies near the stomach, and may be very capacious--much
larger than the stomach in _Loligo vulgaris_--or elongated into a
spiral coil. The simple U-shaped flexure of the alimentary tract, as
seen in fig. 10, is the only important one which it exhibits in the
Cephalopoda. The acini of the large liver of _Nautilus_ are compacted
into a solid reddish-brown mass by a firm membrane, as also is the
case in the Dibranchiata. The liver has four paired lobes in
_Nautilus_, which open by two bile-ducts into the alimentary canal at
the commencement of the intestine. The bile-ducts unite before
entering the intestine. In Dibranchiata the two large lobes of the
liver are placed antero-dorsally (beneath the shell in Decapoda), and
the bile-ducts open into the caecum. Upon the bile-ducts in
Dibranchiata are developed yellowish glandular diverticula, which are
known as "pancreas," though neither physiologically nor
morphologically is there any ground for considering either the
so-called liver or the so-called pancreas as strictly equivalent to
the glands so denominated in the Vertebrata. In _Nautilus_ the
equivalents of the pancreatic diverticula of the Dibranchs can be
traced upon the relatively shorter bile-ducts.

a, The shell.

b, The nuchal plate, identical with the nuchal cartilage of _Sepia_
(see fig. 2, b).

c, The integument covering the visceral hump.

d, The mantle flap or skirt in the dorsal region where it rests
against the coil of the shell.

e, The inferior margin of the mantle-skirt resting on the lip of the
shell represented by the dotted line.

f, The pallial chamber with two of the four gills.

g, The vertically cut median portion of the mid-foot (siphon).

h, The capito-pedal cartilage (see fig. 8).

i, The valve of the siphon.

l, The siphuncular pedicle (cut short).

m, The hood or dorsal enlargement of the annular lobe of the
fore-foot.

n, Tentacles of the annular lobe.

p, Tentacles of inner inferior lobe.

q, Buccal membrane.

r, Upper jaw or beak.

s, Lower jaw or beak.

t, Lingual ribbon.

x, The viscero-pericardial sac.

n.c, Nerve-collar.

oe, Oesophagus.

cr, Crop.

gizz, Gizzard.

int, Intestine.

an, Anus.

nept, Aperture of a nephridial sac.

r.e, Renal glandular masses on the walls of the afferent branchial
veins (see fig. 11).

a.b.v, Afferent branchial vessel.

e.b.v, Efferent branchial vessel.

vt, Ventricle of the heart.]

Posterior salivary glands are not developed in _Nautilus_, but on each
side in the wall of the buccal mass is a gland corresponding to the
anterior salivary gland of the Dibranchiata. No ink-sac is present in
_Nautilus_.

neph, neph, on the right side point to the two nephridia of that
side (the two of the opposite side are not lettered)--each is seen
to have an independent aperture.

x is the viscero-pericardial sac, the dotted line indicating its
backward extension.

visc. per. apert, marks an arrow introduced into the right aperture
of the viscero-pericardial sac.

r.e, r.e, point to the glandular enlarged walls of the afferent
branchial vessels--two small glandular bodies of the kind are seen
to project into each nephridial sac, whilst a larger body of the
same kind depends from each of the four branchial afferent vessels
into the viscero-pericardial sac.

v.c, Vena-cava.

vent, Ventricle of the heart.

ao, Cephalic aorta (the small abdominal aorta not drawn).

a.b.v, Branchial vessel.

e.v.b, Efferent branchial vessel.]

_Coelom, Blood-vascular System and Excretory Organs._--_Nautilus_ and
the other Cephalopoda conform to the general Molluscan characters in
regard to these organs. Whilst the general visceral cavity forms a
lacunar blood-system or series of narrow spaces, connected with the
trunks of a well-developed vascular system, that part of the original
coelom surrounding the heart and known as the Molluscan pericardium is
shut off from this general blood-lymph system, and communicates,
directly in _Nautilus_, in the rest through the renal sacs, with the
exterior. In the Cephalopoda this specialized pericardial cavity is
particularly large, and has been recognized as distinct from the
blood-carrying spaces, even by anatomists who have not considered the
pericardial space of other Mollusca to be thus isolated. The enlarged
pericardium, which may even take the form of a pair of sacs, has been
variously named, but is best known as the viscero-pericardial sac or
chamber. In _Nautilus_ this sac occupies the whole of the
postero-dorsal surface and a part of the antero-dorsal (see fig. 10,
x), investing the genital and other viscera which lie below it, and
having the ventricle of the heart suspended in it. Certain membranes
forming incomplete septa, and a curious muscular band--the
pallio-cardiac band--traverse the sac. The four branchial afferent
veins, which in traversing the walls of the four renal sacs give off,
as it were, glandular diverticula into those sacs, also give off at
the same points four much larger glandular masses, which hang freely
into the viscero-pericardial chamber (fig. 11, r.e). In _Nautilus_ the
viscero-pericardial sac opens to the exterior directly by a pair of
apertures, one placed close to the right and one close to the left
posterior renal aperture (fig. 5, visc.per). This direct opening of
the pericardial sac to the exterior is an exception to what occurs in
all other Mollusca. In all other Molluscs the pericardial sac opens
into the renal organs, and through them or the one renal organ to the
exterior. In _Nautilus_ there is no opening from the
viscero-pericardial sac into the renal sacs. Therefore the external
pore of the viscero-pericardial sac may possibly be regarded as a
shifting of the reno-pericardial orifice from the actual wall of the
renal sac to a position alongside of its orifice. Parallel cases of
such shifting are seen in the varying position of the orifice of the
ink-bag in Dibranchiata, and in the orifice of the genital ducts of
Mollusca, which in some few cases (e.g. _Spondylus_) open into the
renal organs, whilst in other cases they open close by the side of the
renal organs on the surface of the body. The viscero-pericardial sac
of the Dibranchs is very large also, and extends into the dorsal
region. It varies in shape--that is to say, in the extensions of its
area right and left between the various viscera--in different genera,
but in the Decapods is largest. In an extension of this chamber is
placed the ovary of _Sepia_, whilst the ventricle of the heart and the
branchial hearts and their appendages also lie in it. It is probable
that water is drawn into this chamber through the renal sacs, since
sand and other foreign matters are found in it. In all it opens into
the pair of renal sacs by an orifice on the wall of each, not far from
the external orifice (fig. 29, y, y'). There does not seem any room
for doubting that each orifice corresponds to the reno-pericardial
orifice which we have seen in the Gastropoda, and shall find again in
the Lamellibranchia.

A, Part of the dorsal vascular trunk and transverse trunks of a
worm.

B, Ventricle and auricles of _Nautilus_.

C, Of a Lamellibranch, of _Chiton_, or of _Loligo_.

D, Of _Octopus_.

E, Of a Gastropod.

a, Auricle.

v, Ventricle.

ac, Arteria=cephalica=(aorta).

ai, Arteria abdominalis. The arrows show the direction of the
blood-current.]

The circulatory organs, blood-vessels and blood of _Nautilus_ do not
differ greatly from those of Gastropoda. The ventricle of the heart is
a four-cornered body, receiving a dilated branchial efferent vessel
(auricle) at each corner (fig. 11). It gives off a cephalic aorta
anteriorly, and a smaller abdominal aorta posteriorly. The diagram,
fig. 12, serves to show how this simple form of heart is related to
the dorsal vessel of a worm or of an Arthropod, and how by a simple
flexure of the ventricle (D) and a subsequent suppression of one
auricle, following on the suppression of one branchia, one may obtain
the form of heart characteristic of the anisopleurous Gastropoda
(excepting the Aspidobranchia). The flexed condition of the heart is
seen in _Octopus_, and is to some extent approached by _Nautilus_, the
median vessels not presenting that perfect parallelism which is shown
in the figure (B). The most remarkable feature presented by the heart
of _Nautilus_ is the possession of four instead of two auricles, a
feature which is simply related to the metamerism of the branchiae. By
the left side of the heart of _Nautilus_, attached to it by a
membrane, and hanging loosely in the viscero-pericardial chamber, is
the pyriform sac of Owen. This has been shown to be the rudimentary
left oviduct or sperm-duct, as the case may be (E.R. Lankester and
A.G. Bourne), the functional right ovi-sac and its duct being attached
by a membrane to the opposite side of the heart.

The cephalic and abdominal aortae of _Nautilus_ appear, after running
to the anterior and posterior extremes of the animal respectively, to
open into sinus-like spaces surrounding the viscera, muscular masses,
&c. These spaces are not large, but confined and shallow. Capillaries
are stated to occur in the integument. In the Dibranchs the arterial
system is very much more complete; it appears in some cases to end in
irregular lacunae or sinuses, in other cases in true capillaries which
lead on into veins. An investigation of these capillaries in the light
of modern histological knowledge is much needed. From the sinuses and
capillaries the veins take origin, collecting into a large median
trunk (the vena cava), which in the Dibranchs as well as in _Nautilus_
has a ventral (postero-ventral) position, and runs parallel to the
long axis of the animal. In _Nautilus_ this vena cava gives off at the
level of the gills four branchial afferent veins (fig. 11, v.c.),
which pass into the four gills without dilating. In the Dibranchs at a
similar position the vena cava gives off a right and a left branchial
afferent vein, each of which, traversing the wall of the corresponding
renal sac and receiving additional factors, dilates at the base of the
corresponding branchial plume, forming there a pulsating sac--the
branchial heart. Attached to each branchial heart is a curious
glandular body, which may possibly be related to the larger masses
(fig. 11, r.e) which depend into the viscero-pericardial cavity from
the branchial afferent veins of _Nautilus_. From the dilated branchial
heart the branchial afferent vessel proceeds, running up the adpallial
face of the gill-plume. From each gill-plume the blood passes by the
branchial efferent vessels to the heart, the two auricles being formed
by the dilatation of these vessels.

The blood contains the usual amoeboid corpuscles, and a diffused
colouring matter--the haemocyanin of Fredericque--which has been found
also in the blood of _Helix_, and in that of the Arthropods _Homarus_
and _Limulus_. It is colourless in the oxidized, blue in the
deoxidized state, and contains copper as a chemical constituent.

The renal sacs and renal glandular tissue are closely connected with
the branchial advehent vessels in _Nautilus_ and in the other
Cephalopoda. The arrangement is such as to render the typical
relations and form of a renal tube difficult to trace. In accordance
with the metamerism of _Nautilus_ already noticed, there are two pairs
of renal organs. Each assumes the form of a sac opening by a pore to
the exterior. As is usual in renal tubes a glandular and a
non-glandular portion are distinguished in each sac; these portions,
however, are not successive parts of a tube, as happens in other
cases, but they are localized areae of the wall of the sac. The
glandular renal tissue is, in fact, confined to a tract extending
along that part of the sac's wall which immediately invests the great
branchial afferent vein. The vein in this region gives off directly
from its wall a complete herbage of little venules, which branch and
anastomose with one another, and are clothed by the glandular
epithelium of the renal sac. The secretion is accumulated in the sac
and passed by its aperture to the exterior. Probably the nitrogenous
excretory product is very rapidly discharged; in _Nautilus_ a
pink-coloured powder is found accumulated in the renal sacs,
consisting of calcium phosphate. The presence of this phosphatic
calculus by no means proves that such was the sole excretion of the
renal glandular tissue. In _Nautilus_ a glandular growth like that
rising from the wall of the branchial vessel into its corresponding
renal sac, but larger in size, depends from each branchial afferent
vessel into the viscero-pericardial sac and forms the pericardial
gland--probably identical with the "appendage" of the branchial hearts
of Dibranchs.

The chief difference, other than that of number, between the renal
organs of the Dibranchs and those of _Nautilus_, is the absence of the
accessory growths depending into the viscero-pericardial space just
mentioned, and, of more importance, the presence in the former of a
pore leading from the renal sac into the viscero-pericardial sac (y,
y' in fig. 29). The external orifices of the renal organs are also
more prominent in Dibranchs than in _Nautilus_, being raised on
papillae (np in fig. 29; r in fig. 25). In _Sepia_ the two renal sacs
give off each a diverticulum dorsalwards, which unites with its fellow
and forms a great median renal chamber, lying between the ventral
portions of the renal organs and the viscero-pericardial chamber. In
_Loligo_ the fusion of the two renal organs to form one sac is still
more obvious, since the ventral portions are united. In _Octopus_ the
renal sacs are quite separate.

_Gonads and Genital Ducts._--In _Nautilus_it has been shown by E. Ray
Lankester and A.G. Bourne that the genital ducts of both sexes are
paired right and left, the left duct being rudimentary and forming the
"pyriform appendage," described by Sir R. Owen as adhering by
membranous attachment to the ventricle of the heart, and shown by W.
Keferstein to communicate by a pore with the exterior. The ovary
(female gonad) or the testis (male gonad) lies in _Nautilus_, as in
the Dibranchs, in a distinct cavity walled off from the other viscera,
near the centro-dorsal region. This chamber is formed by the coelomic
or peritoneal wall; the space enclosed is originally part of the
coelom, and in _Sepia_ and _Loligo_ is, in the adult, part of the
viscero-pericardial chamber. In _Octopus_ it is this genital chamber
which communicates by a right and a left canal with the renal sac, and
is the only representative of pericardium. The ovary or testis is
itself a growth from the inner wall of this chamber, which it only
partly fills. In _Nautilus_ the right genital duct, which is
functional, is a simple continuation to the pore on the postero-dorsal
surface of the membranous walls of the capsule in which lies the ovary
or the testis, as the case may be. The gonad itself appears to
represent a single median or bilateral organ.

The ovary forms a large projection into the genital coelom, and the
coelomic epithelium is deeply invaginated into the mass of the gonad,
so as to constitute an ovarian cavity communicating with the coelom by
a narrow aperture. The ova originate in the epithelium, migrate below
it and then, as they enlarge, project into the ovarian cavity, pushing
the epithelium before them. Each ovum is surrounded by a follicular
epithelium which is nourished by numerous blood-vessels, and which
penetrates into the surface of the ovum in numerous folds. When
mature, the ovum is contained in a membrane or chorion with a
micropyle, and escapes by dehiscence of the follicle into the genital
coelom and duct. In its passage to the exterior the ovum passes a
glandular structure on the wall of the genital capsule, which probably
secretes the gelatinous substance enclosing the eggs. In addition to
this internal gland there are other accessory glands, which are not
related to the genital duct or sac but are differentiations of the
wall of the pallial cavity, and occur on the inner wall of the pallium
in _Nautilus_, on the somatic wall in Dibranchiata. In _Nautilus_ they
form a continuous mass. These produce the external envelopes of the
eggs.

In the male the testis is a specialized portion of the wall of the
genital coelom, and has a structure comparable to that of the ovary.
The spermatozoa pass through an orifice from the cavity of the testis
to the genital capsule, and thence to the spermiduct. The spermiduct
is provided with a glandular pouch, and opens into a terminal
reservoir known as Needham's sac or the spermatophore sac. The
function of this pouch is to form the spermatophore, which is an
elastic tube formed of structureless secretion and invaginated into
itself. The deeper part contains the spermatozoa, the external part is
called the connective, and is usually much contracted and spirally
coiled. When the spermatophore is expelled into the water the
connective is extended and evaginated, and the sac containing the
sperms bursts. In _Nautilus_ the spermatophore when uncoiled is a
little over 30 mm. in length. These spermatophores are somewhat
similar to those formed in certain pulmonate Gastropods.

The eggs are laid shortly after copulation. In _Nautilus_ they are
laid separately, each being about 4 cm. long and contained in two
thick shells, the outer of which is partly open.

t, t, Ganglion-like enlargements on nerves passing from the pedal
ganglion to the inner series of tentacles.

t', Nerves to the tentacles of the outer or annular lobe.

b, Pedal ganglion-pair

a, Cerebral ganglion-pair.

c, Pleuro-visceral ganglionic band (fused pleural and visceral
ganglion-pairs).

d, Genital ganglion placed on the course of the large visceral
nerve, just before it gives off its branchial and its osphradial
branches.

m, Nerves from the pleural ganglion to the mantle-skirt.]

_Nervous System._--_Nautilus_, like the other Cephalopoda, exhibits a
great concentration of the typical Molluscan ganglia, as shown in fig.
13. The ganglia take on a band-like form, and are but little
differentiated from their commissures and connectives--an archaic
condition reminding us of _Chiton_. The special optic outgrowth of the
cerebral ganglion, the optical ganglion (fig. 13, o), is
characteristic. The cerebral ganglion-pair (a) lying above the
oesophagus is connected with two suboesophageal ganglion-pairs, of
band-like form. The anterior of these is the pedal _b, b_, and
supplies the circumoral lobes and tentacles, and the funnel, a fact
which proves the pedal origin of these organs. The hinder band is the
visceral and pleural pair fused; from its pleural portion nerves pass
to the mantle, from its visceral portion nerves to the branchiae and
genital ganglion (fig. 13, d), and in immediate connexion with the
latter is a nerve to the osphradium or olfactory papilla. A labial
commissure arises by a double root from the cerebral ganglia and gives
off a stomatogastric commissure, which passes under the pharynx
immediately behind the radula and bears a buccal ganglion on either
side.

_Special Sense-Organs._--_Nautilus_ possesses a pair of osphradial
papillae (fig. 4, olf) corresponding in position and innervation to
Spengel's organ placed at the base of the ctenidia (branchiae) in all
classes of Mollusca. This organ has not been detected in other
Cephalopoda. _Nautilus_ possesses other olfactory organs in the region
of the head. Just below the eye is a small triangular process (not
seen in our figures), having the structure of a shortened and
highly-modified tentacle and sheath. By A. Valenciennes, who is
followed by W. Keferstein, this is regarded as an olfactory organ. The
large nerve which runs to this organ originates from the point of
juncture of the pedal with the optic ganglion. The lamelliform organ
upon the inner inferior tentacular lobe of _Nautilus_ is possibly also
olfactory in function. In Dibranchs behind the eye is a pit or open
canal supplied by a nerve corresponding in origin to the olfactory
nerve of _Nautilus_ above mentioned. Possibly the sense of taste
resides in certain processes within the mouth of _Nautilus_ and other
Cephalopoda.

The otocysts of _Nautilus_ were discovered by J.D. Macdonald. Each
lies at the side of the head, ventral to the eye, resting on the
capito-pedal cartilage, and supported by the large auditory nerve
which apparently arises from the pedal ganglion but originates in the
cerebral. It has the form of a small sac, 1 to 2 mm. in diameter, and
contains whetstone-shaped crystals, such as are known to form the
otoliths of other Mollusca.

The eye of _Nautilus_ is among the most interesting structures of that
remarkable animal. No other animal which has the same bulk and general
elaboration of organization has so simple an eye as that of
_Nautilus_. When looked at from the surface no metallic lustre, no
transparent coverings, are presented by it. It is simply a slightly
projecting hemispherical box like a kettle-drum, half an inch in
diameter, its surface looking like that of the surrounding integument,
whilst in the middle of the drum-membrane is a minute hole (fig. 3,
u). Sir R. Owen very naturally thought that some membrane had covered
this hole in life, and had been ruptured in the specimen studied by
him. It, however, appears from the researches of V. Hensen that the
hole is a normal aperture leading into the globe of the eye, which is
accordingly filled by sea-water during life. There is no dioptric
apparatus in _Nautilus_, and in place of refracting lens and cornea we
have actually here an arrangement for forming an image on the
principle of "the pin-hole camera." There is no other eye known in the
whole animal kingdom which is so constructed. The wall of the
eye-globe is tough, and the cavity is lined solely by the naked
retina, which is bathed by sea-water on one surface and receives the
fibres of the optic nerve on the other (see fig. 14, A). As in other
Cephalopods (e.g. fig. 33, Ri, Re, p), the retina consists of two
layers of cells, separated by a layer of dark pigment. The most
interesting consideration connected with this eye of _Nautilus_ is
found when the further facts are noted--(1) that the elaborate
lens-bearing eyes of Dibranchiata pass through a stage of development
in which they have the same structure as the eye of
_Nautilus_--namely, are open sacs (fig. 34); and (2) that amongst
other Mollusca examples of cephalic eyes can be found which in the
adult condition are, like the eye of _Nautilus_ and the developing eye
of Dibranchs, simple pits of the integument, the cells of which are
surrounded by pigment and connected with the filaments of an optic
nerve. Such is the structure of the eye of the limpet (_Patella_), and
in such a simple eye we obtain the clearest demonstration of the fact
that the retina of the Molluscan cephalic eye, like that of the
Arthropod cephalic eye and unlike that of the vertebrate myelonic eye,
is essentially a modified area of the general epiderm, and that the
sensitiveness of its cells to the action of light and their relation
to nerve-filaments is only a specialization and intensifying of a
property common to the whole epiderm of the surface of the body. What,
however, strikes us as especially remarkable is that the simple form
of a pit, which in _Patella_ serves to accumulate a secretion which
acts as a refractive body, should in _Nautilus_ be glorified and
raised to the dignity of an efficient optical apparatus. In all other
Mollusca, starting as we may suppose from the follicular or pit-like
condition, the eye has proceeded to acquire the form of a _closed_
sac, the cavity of the closed vesicle being then filled partially or
completely by a refractive body (lens) secreted by its walls (fig. 14,
B). This is the condition attained in most Gastropoda. It presents a
striking contrast to the simple Arthropod eye, where, in consequence
of the existence of a dense exterior cuticle, the eye does not form a
vesicle, and the lens is always part of that cuticle.

A, _Nautilus_ (and _Patella_).

B, Gastropod (_Limax_ or _Helix_).

C, Dibranchiate Cephalopod (Oigopsid).

Pal, Eyelid (outermost fold).

Co, Cornea (second fold).

Ir, Iris (third fold).

Int 1,2,3,4, Different parts of the integument.

l, Deep portion of the lens.

l^1, Outer portion of the lens

Co.ep, Ciliary body.

R, Retina.

N.op, Optic nerve.

G.op, Optic ganglion.

x, Inner layer of the retina.

N.S., Nervous stratum of the retina. (From Balfour, after
Grenacher.)]

The development of _Nautilus_ is still entirely unknown. Dr Arthur
Willey, during his sojourn in the East Indies, made special efforts to
obtain fertilized eggs, both by offering rewards to the native
fishermen and collectors and by keeping the living adults in
captivity, but without success.

_Phylogeny and Classification._--As _Nautilus_ is the only living
genus of the Tetrabranchiata, our knowledge of all the rest is based
upon the study of their fossil shells. A vast number of species of
shell similar in structure to that of _Nautilus_ are known, chiefly
from Primary and Secondary formations. These are divided into two
sub-orders by differences in the form and structure of the initial
chamber. In the Nautiloidea this chamber has the form of an obtuse
cone, on the apex of which is a slit-like mark or cicatrix, elongated
dorso-ventrally and placed opposite to the blind end of the siphuncle,
which indents the front wall of the initial chamber but does not enter
its cavity. In the Ammonoidea, on the other hand, the initial chamber
is inflated, and is spheroidal, oval or pyriform in shape, with no
cicatrix, and separated from the first air-chamber by a constriction.
The siphuncle also commences with a dilatation which deeply indents
the front wall of the initial chamber, called the protoconch, but does
not penetrate into its cavity. Munier-Chalmas has shown that the
cavity of the protoconch is traversed by a tubular organ, the
"prosiphon," which does not communicate with the true siphuncle, the
place of which it is supposed to take in the early life of the animal.
It is generally held, as suggested by Alpheus Hyatt, that the initial
chamber of the Nautiloidea corresponds not to the protoconch of the
Ammonoids, but to the second chamber of the latter, and that there
existed in the young Nautiloids a true initial chamber, a protoconch
which was either uncalcified or deciduous. The shell of the living
nautilus does not decide this question, as its early stages are
unknown, and there is a little vacuity in the centre of the spirally
coiled shell which may have been originally occupied by the true
protoconch.

The septa in the Nautiloidea are generally concave towards the
aperture of the shell, their curvature therefore directed backwards
(fig. 1); in the Ammonoidea, on the other hand, the convexity is
usually towards the aperture, the curvature therefore directed
forwards. The lines along which the edges of the septa are united to
the shell are known as "sutures," and these in the Nautiloidea are
simply curved or slightly lobed, whereas in the Ammonoidea they are
folded in various degrees of complexity; the projections of the suture
towards the mouth of the shell are called saddles, those in the
opposite direction lobes. The siphuncle in the _Nautilus_ pierces the
centres of the septa, and in fossil Nautiloids it is usually central
or sub-central. In a few cases it is marginal, and in that case may be
external, i.e. ventral, or internal, i.e. dorsal. In Ammonoids the
siphuncle is always marginal, and usually external. Its walls in the
living _Nautilus_ are strengthened by the deposit of calcareous
granules, and in some fossil forms the wall is completely calcified.
But this proper calcified wall is quite distinct from calcareous tubes
surrounding the siphuncle, which are developed from the septa. In the
pearly nautilus each septum is prolonged backwards at the point where
it is pierced by the siphuncle, forming a shelly tube somewhat like
the neck of a bottle. In many fossil forms these septal necks are
continued from the septum from which they arise to the next, so that
the siphuncle is enclosed in a complete secondary calcareous tube. In
the majority of Nautiloids the septal necks are directed backwards,
and they are said to be retrosiphonate. In the majority of the
Ammonoids the septal necks are continued forwards from the septa to
which they belong, and such forms are termed prosiphonate.

The Tetrabranchiata were most abundant in the Palaeozoic and Mesozoic
periods. The Nautiloidea are the most ancient, appearing first in the
Upper Cambrian, the genera being most numerous in the Palaeozoic
period, and comparatively few surviving into the Secondary. On the
other hand, the Ammonoidea are scarce in Palaeozoic formations, being
represented in deposits earlier than the Carboniferous only by
comparatively simple types, such as _Clymenia_ and _Goniatites_. In
the Secondary period Ammonoids were very abundant, both in genera and
species and in individuals, and with few local exceptions none are
known to have survived even to the commencement of the Tertiary. In
the widest sense the genus _Nautilus_ has existed since the Ordovician
(Silurian) period, but the oldest types are not properly to be placed
in the same genus as the existing form. Even with this qualification
the genus is very ancient, shells very similar to those of the living
_Nautilus_ being found in the Upper Cretaceous.

It has been maintained by some zoologists that the Ammonoidea were
Dibranchiate, though it would not follow from this that the shell was,
therefore, internal. They are, however, generally classed with the
Tetrabranchiata, and the absence of all evidence of the possession of
an ink-sac is in favour of this view. There can be little doubt that
they gave rise to the Dibranchiata.

About 2500 fossil species are included in the Nautiloidea, but only a
few species of the genus _Nautilus_ survive. Some of the fossil forms
are very large, the shell reaching a length of 2 metres, or 6 ft. 6
in. Of the Ammonoidea more than 5000 species have been described, and
some of the coiled forms are 70 cm., or nearly 2 ft. 6 in. in
diameter.

Associated with various forms of Ammonoids there have been found
peculiar horny or calcified plates, sometimes contained within the
body-chamber of the shell, sometimes wholly detached. The most typical
form of these structures has been named _aptychus._ It consists of two
bilaterally symmetrical halves, of somewhat semicircular shape, and
attached to one another by their straight inner margins, like a pair
of doors. In some cases the aptychus is thin and horny, but more often
it is thick and calcified, in which case the principal layer has a
peculiar cellular structure. The surface may be smooth or sculptured,
and one side is usually marked by concentric lines of growth. Another
type is similar, except that the two halves are united in the middle
line; bodies of this character are called _synaptychus_; they occur in
the body-chamber of species of _Scaphites_. Another form called
_anaptychus_ consists of a thin horny undivided plate which is
concentrically striated. This is associated with species of
_Ammonites_ and _Goniatites_.

Many theories have been proposed in explanation of these structures.
According to Sir Richard Owen, the aptychus is an operculum developed
in a part of the body corresponding to the hood of _Nautilus_. E. Ray
Lankester suggested that the double plate was borne on the surface of
the nidamental gland, with the form and sculpturing of which in
_Nautilus_ it closely agrees. On this view the aptychus would occur
only in females. The most recent view is that these structures could
not have been opercula because of their constant position inside the
body-chamber, and that they were not external secretions at all, but a
calcified internal cartilage situated at the base of the funnel.

_Classification of Tetrabranchiata._--Cephalopoda in which the mantle
is entirely enclosed by a multilocular siphunculated shell, which may
or may not be coiled. Only the last compartment of the shell occupied
by the body of the animal. Numerous pedal tentacles around the mouth,
which are retractile within sheaths. Halves of the funnel not united.
Two pairs of ctenidia, and two pairs of renal tubes without
reno-pericardial apertures. Pericardium opens directly to exterior.
Cephalic cartilage wholly ventral. Optic vesicles with apertures,
without crystalline lens.

_Sub-order 1. Nautiloidea_.--Initial chamber not inflated, with
dorso-ventral cicatrix at extremity.

Fam. 1. _Orthoceratidae_. Shell straight or slightly curved, with a
simple aperture, large terminal chamber and cylindrical siphuncle.
_Orthoceras_, Silurian to Trias. _Baltoceras_, Silurian.

Fam. 2. _Actinoceratidae_. Shell straight or slightly curved, with
wide siphuncle contracted at level of septa. _Actinoceras_, Silurian
to Carboniferous. _Discosorus_, Silurian. _Huronia_, Silurian.
_Loxoceras_, Silurian to Carboniferous.

Fam. 3. _Endoceratidae_. Shell straight, with wide margina
siphuncle, necks produced into tubes fitting into one another.
_Endoceras_, Silurian.

Fam. 4. _Gomphoceratidae_. Shell globular, straight or arcuate,
aperture contracted. _Gomphoceras_, Silurian. _Phragmoceras_,
Silurian.

Fam. 5. _Ascoceratidae_. Shell straight, ampulliform, summit
truncate, terminal chamber extending nearly whole length of shell
ventrally. _Ascoceras_, Silurian. _Glossoceras_, Silurian.

Fam. 6. _Poterioceratidae_. Shell straight or curved, fusiform,
aperture simple, siphuncle contracted at septa. _Poterioceras_,
Silurian to Carboniferous. _Streptoceras_, Silurian.

Fam. 7. _Cyrtoceratidae_. Shell slightly curved, aperture simple,
siphuncle wide, septa approximated. _Cyrtoceras_, Devonian.

Fam. 8. _Lituitidae_. Shell coiled in one plane with the terminal
part uncoiled, aperture contracted. _Lituites_, Silurian.
_Ophidioceras_, Silurian.

Fam. 9. _Trochoceratidae_. Shell helicoidally coiled, dextral or
sinistral, the last whorl generally uncoiled. _Trochoceras_,
Devonian. _Adelphoceras_, Devonian.

Fam. 10. _Nautilidae_. Shell coiled in one plane, aperture wide and
simple, siphuncle central. _Nautilus_, recent. _Trocholites_,
Silurian. _Gyroceras_, Silurian to Carboniferous. _Hercoceras_,
Silurian. _Ptenoceras_, Devonian. _Discites_, Carboniferous.

Fam. 11. _Bactritidae_. Shell straight, conical, siphuncle narrow
and marginal, necks long, infundibuliform, sutures undulating.
_Bactrites_, Silurian and Devonian.

_Sub-order 2. Ammonitoidea_,--Initial chamber spheroidal; siphuncle
narrow and simple; septa convex towards aperture; sutures complex.

_Tribe 1. Retrosiphonata_.--Siphuncular necks projecting behind the
septa as in Nautiloidea. Sutures form simple undulations. Occur
exclusively in Palaeozoic strata from Devonian upwards.

Fam. 1. _Goniatitidae_. Shell nautiloid, with simple sutures and
ventral siphuncle. _Goniatites_, Devonian and Carboniferous.
_Anarcestes_, Devonian.

Fam. 2. _Clymeniidae_. Shell nautiloid, with simple sutures,
siphuncle dorsal, that is, internal. _Clymenia_, Upper Devonian.

_Tribe 2. Prosiphonata._--Siphuncular necks projecting in front of the
septa. Sutures form deeply indented lobes and saddles.

Fam. 1. _Arcestidae_. Globular and smooth or nearly smooth, with
reduced umbilicus, terminal chamber very deep, an aptychus present.
_Popanoceras_, Permian. _Cyclolobus_, Permian, _Arcestes_, Trias.
_Lobites_, Trias.

Fam. 2. _Tropitidae_. Shells globular, but having radiating and
tuberculated costae. _Thalassoceras_, Permian. _Tropites_, Trias.
_Sibirites_, Trias.

Fam. 3. _Ceratitidae_. Shells coiled, with a large umbilicus,
terminal chamber short, sutures with simple saddles. _Trachyceras_,
Upper Trias. _Ceratites_, Trias. _Dinarites_, Trias.

Some genera with helicoidal shells are related to these coiled forms,
viz. _Cochloceras_, Trias; also some straight forms, e.g.
_Rhab-doccras_, Trias.

Fam. 4. _Pinacoceratidae_. Shell compressed, smooth, terminal
chamber short, sutures very complicated, convex. _Pinacoceras_,
Trias.

Fam. 5. _Phylloceratidae_. Shell coiled, the whorls overlapping each
other, sutures formed of numerous lobes and saddles. _Phytloceras_,
Jurassic.

Fam. 6. _Lytoceratidae_. Shell discoid, whorls loosely united or
uncoiled, sutures deeply indented, but with only three saddles and
lobes. _Lytoceras_, Jurassic and Cretaceous. _Macroscaphites_,
Cretaceous. _Reunites_, Cretaceous. _Ptychoeeras_, Cretaceous.
_Turrilites_, Cretaceous. _Baculites_, Cretaceous.

Fam. 7. _Ammonitidae_. Shell coiled, with narrow whorls which do not
embrace one another, aperture simple, a horny anaptychus present.
_Ammonites_, Jurassic. _Arietites_, Jurassic. _Aegoceras_, Lias.

Fam. 8. _Harpoceratidae_. Shell discord and flattened, with a
carinated border, aperture provided with lateral projections, a
calcareous aptychus, formed of two pieces. _Harpoceras_, Jurassic.
_Oppelia_, Jurassic. _Lissoceras_, Jurassic and Cretaceous.

Fam. 9. _Amaltheidae_. Shell flattened, with a prominent carina
continued anteriorly into a rostrum. _Amaltheus_, Lias.
_Cardioceras_, Jurassic. _Schloenbachia_, Cretaceous.

Fam. 10. _Stephanoceratidae_. Shell not carinated, but with
radiating costae, which are often bifurcated, aperture often with
lateral projections which contract it, aptychus formed of two
pieces. _Stephanoceras, Morphoceras, Pensphinctes, Peltoceras_,
Jurassic. _Hoplites_, Cretaceous. _Acanthoceras_, Cretaceous.
_Cosmoceras_, Jurassic. Various more or less uncoiled forms are
related to this family, viz. _Scaphites, Crioceras_, Cretaceous.

ORDER 2. DIBRANCHIATA (= Holosiphona, Acetabulifera)

_Characters_.--Cephalopoda in which the inflected margins of the epipodia are fused so as to form a complete tubular siphon (fig. 24, i). The circumoral lobes of the fore-foot carry suckers disposed upon them in rows, _not_ tentacles (see figs. 15, 24). There is a single pair of typical ctenidia (fig. 25) acting as gills (hence Dibranchiata), and a single pair of renal organs, opening by apertures right and left of the median anus (fig. 25, r) and by similar internal pores into the pericardial chamber, which consequently does not open directly to the surface as in _Nautilus_. The oviducts are sometimes paired right and left (Octopoda, Oigopsida), sometimes that of one side only is developed (Myopsida). The sperm-duct is always single except, according to W. Keferstein, in _Eledone moschata._

A plate-like shell is developed in a closed sac formed by the mantle (figs 20, 21), except in the Octopoda, which have none, and in _Spirula_ (fig. 17, D) and the extinct _Belemnitidae_, &c., which have a small chambered shell resembling that of _Nautilus_ with or without the addition of plate-like and cylindrical accessory developments (fig. 17, A, C, fig. 19).

The pair of cephalic eyes are highly-developed vesicles with a refractive lens (fig. 33), cornea and lid-folds,--the vesicle being in the embryo, an open sac like that of _Nautilus_ (fig. 34). Osphradia are not present, but cephalic olfactory organs are recognized. One or two pairs of large salivary glands with long ducts are present. An ink-sac formed as a diverticulum of the rectum and opening near the anus is present in all Dibranchiata (fig. 25, t), and has been detected even in the fossil _Belemnitidae._ Branchial hearts are developed on the two branchial afferent blood-vessels (fig. 28, _vc'_, _vi_).

A, _Cheiroteuthis Veranyi_, d'Orb. (from the Mediterranean).

B, _Thysanoteuthis rhombus_, Troschel (from Messina).

C, _Loligopsis cyclura_, Fér. and d'Orb. (from the Atlantic Ocean).]

A, _Conoteutliis dupiniana_, d'Orb. (from the Neocomian of France).

B, Shell _Sepia orbigniana_. Fér. (Mediterranean).

C, Shell of _Spirulirostra Bellardii_, d'Orb. (from the Miocene of
Turin). The specimen is cut so as to show in section the chambered
shell and the laminated "guard" deposited upon its surface.

D, Shell of _Splrula laevis_, Gray (New Zealand).]

In the Dibranchiata the shell shows various stages of degeneration, culminating in its complete disappearance in _Octopus_. As in other Mollusca, there is a tendency in Cephalopods for the mantle to extend over the outside of the shell from its edges, and when these secondary mantle-folds entirely cover the shell and meet or fuse together the shell is surrounded by the mantle both externally and internally, and is said to be internal, though it remains always a cuticular structure external to the epidermis. This procebs is generally accompanied by a reduction of the size of the shell in comparison with that of the body, so that the relations of the two are gradually reversed, the body outgrows its house and instead of the mantle being enclosed by the shell, the shell is enclosed by the mantle. The earliest stage of this process is shown in the recent _Spirula,_ though it is perhaps not impossible that in some of the later fossil Ammonoids the shell was becoming more and more internal. The shell of _Spirula_ (fig. 18) is coiled somewhat like that of _Nautilus_, but the coils are not in contact, the direction of the coil is endogastric or ventral instead of exogastric, and the shell is very much smaller than the body. Like that of _Nautilus_ it is divided by septa and traversed by a siphuncle. The relation of the animal to the terminal chamber is as in _Nautilus,_ but the body extends far beyond the aperture, and folds of the mantle grow up over the shell and cover it everywhere except part of the dorsal and ventral surfaces.

FIG. 18.--_Spirula._

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]

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_.

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.

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).

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.]

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.

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.