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.
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.
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