Fig. 26.—Diagram representing a vertical approximately medianantero-posterior section of Sepia officinalis (from a drawing by A.G.Bourne). The lettering corresponds with that of fig. 10, with whichthis 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.

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.

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.

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.