The class Cephalopoda comprises Mollusca in which there is a distinct head, and a toothed “tongue” or “odontophore,” whilst the hinder extremity of the body is inclosed in a muscular mantle-sac, which may or may not secrete an external shell. The mouth is placed near the centre of the “foot,” and the margins of this structure are split up into 8 (Octopod Cuttle-fishes), 10 (Decapod Cuttle-fishes), or numerous (Pearly Nautilus) muscular processes, or “arms.” The lateral margins of the foot (“epipodia ”) constitute, by apposition or fusion, a muscular tube (the “funnel ”) through which the effete water of respiration is expelled.

The class Cephalopoda is divided into the two great orders of the Tetrabranchiata and the Dibranchiata.

The Tetr abranchiate order comprises only the living species, or varieties, of the Pearly Nautilus (Nautilus pom- pilius), along with a vast number of fossil forms, and is characterized by the possession of an external, many-chambered, siphunculate shell; by the presence of numerous arms, which are devoid of suckers; by the possession of four branchiae; by the absence of an ink-sac; and by the fact that the “funnel” does not form a complete tube.

The order Dibranchiata, with which alone we are concerned here, comprises the true cuttle-fishes, in which there are either 8 or 10 arms, provided with suctorial discs; there is no external shell, or, in the single case in which such a structure is present (the female Argonaut), it is single-chambered, and is not secreted by the mantle; there are only two branchiae; an ink-sac is present; and the “funnel” forms a complete tube.

The body of a cuttle-fish is symmetrical, and is'divisible into an anterior cephalic portion (prosoma) and a posterior abdominal portion (metasoma). The former of these is developed into a distinct head, furnished on its sides with large and prominent eyes, and having the mouth in the centre of its anterior surface, surrounded by eight or ten “arms.” The latter incloses the various viscera, and is enveloped in an integumentary sac, which corresponds to the “mantle” (“pallium ”) of the Gasteropods and Lamellibranchs. The mantle-sac is formed by the coalescence of the two pallial lobes along the ventral surface of the body, and it is attached directly to the metasoma along the dorsal surface, whilst it is free inferiorly, and incloses a space (the “pallial chamber ”) which contains the gills, and into which the intestine and the ducts of the generative organs and ink-sac open.

The integument of the cuttle-fishes consists of several layers, of which the most important is one which corresponds to the lowermost layer of the epidermis, and which is distinguished by the possession of numerous large-sized cells filled with pigment-granules (“chromatophores ”). These pigment-cells are capable of expanding and contracting in their dimensions, and of altering in shape, and below them are other flattened nucleated refracting cells, which co-operate with the former in the production of the marvellous play of changing colours which the cuttle-fishes exhibit under excitement or irritation.

The muscular system of the cuttle-fishes is well developed, the fibres being long and spindle-shaped, and only in certain situations (e.y., in the branchial hearts) transversely striated. The mantle is in all highly muscular, but the most important muscular organs are the “arms,” the “tentacles,” the “fins,” and the “funnel.” The “arms” are long processes produced by the splitting up of the antero-lateral margins of the foot, and the mouth is placed in the centre of their bases. In all the cuttle-fishes eight arms are present, but the so-called Decapods have in addition the “tentacles,” whilst the Octopods are devoid of these supplementary processes. The arms are longer or shorter pointed processes, formed principally of longitudinal muscles, with well-marked, radial bundles of fibres, and having a nerve and an artery occupying the axis of each. They are placed symmetrically round the mouth, forming a dorsal pair, a ventral pair, and two lateral pairs on each side; and their bases are connected by an inter-brachial membrane, which in some instances (Girrhoteuthis) extends nearly to their points. On the inner surface of the arms are placed the suckers (“acetabula ”), in the form of muscular cup-like discs, which may be sessile or stalked, and which are arranged generally in one or two, or rarely in four, rows. Each acetabulum consists of a cup, the margin of which is formed by a muscular ring, sometimes strengthened by a horny girdle (which may be smooth or may be produced into teeth), whilst its centre is occupied by an elevated papilla composed mainly of radial muscular fibres. When the sucker is applied to any object, the contraction of the muscular fibres causes the depression of this muscular papilla, and creates a partial vacuum, thus enabling each sucker to act as a most efficient organ of prehension—their action being sometimes supplemented (as in Onychoteuthis) by the conversion of the central papilla into a horny hook. The so-called “tentacles” of the Decapod Cuttle-fishes resemble the true arms in structure, but are very much longer, and only carry suckers on their swollen and clubshaped extremities. They are placed on the ventral surface of the animal, between the third and fourth pairs of arms (counting from the middle line of the back); they may or may not be retractile into pouches placed below the eyes; and they may attain a length many times greater than that of the body itself. The tentacles are organs of prehension, and the arms are, in addition, employed by the animal in locomotion, enabling it to walk head downwards, at the bottom of the sea, or, when webbed, to swim through the water in a retrograde manner. One of the arms of the male cuttle-fishes, as will subsequently appear, is also more or less largely engaged in the work of reproduction, and may for this reason be greatly modified.

The sides of the body in all the Decapod Cuttle-fishes, and in a few of the Octopods (e.y., Pinnoctopus), are more or less extensively produced into muscular expansions or fins, supported internally by a cartilaginous basis. These fins are employed by the animal in swimming head foremost, and they may extend along the whole length of the metasoma (as in Sepia), or they may be confined to the hinder end of the body (as in Loligo, Cheiroteuthis, Onychotcuthis, &c.)

The “funnel” of the cuttle-fishes is a muscular tube, formed by the union of the “epipodia,” and placed on the lower surface of the body, with its anterior extremity projecting beyond the mantle, whilst it opens posteriorly into the pallia! chamber. It serves for the extrusion (by means of the outgoing respiratory currents) of the undigested portions of the food and of the excretions of the kidneys and ink-sac; whilst the water which has passed over the gills is expelled through it in a succession of jets, subserving in this way the secondary purpose of driving the animal backwards through the water.

As regards the digestive system, the mouth is placed centrally, surrounded by the bases of the arms; and it conducts into a powerfully muscular buccal cavity, in which are contained two strong horny jaws and a well-developed “tongue.” The jaws, or “mandibles,” are purely horny (not partially calcareous, as in the Pearly Nautilus), and they have very much the form of a parrot’s beak, working vertically, the lower one projecting most and receiving the upper mandible within it in the act of biting. The so-called “tongue” is a muscular organ, part of which is covered with numerous papillae, and is apparently an organ of taste; whilst another portion is developed into a lingual ribbon essentially similar in its structure to the “odontophore” of the Gasteropoda. The oesophagus— sometimes simple (Decapoda), sometimes provided with proventricular or crop-like dilatations (Octopoda)— conducts from the buccal chamber to the stomach, the latter organ being of large size, highly muscular, of a generally rounded shape, and having appended to its pyloric extremity a capacious diverticulum, into which the bile-ducts open. Into the oesophagus open the ducts of one or two pairs of salivary glands; and the liver is of large size and highly developed, whilst certain glandular structures which pour their secretion into the bile-ducts are believed to represent the pancreas. The intestine is usually short, mostly of nearly uniform calibre, straight or slightly convoluted, and terminates in an anal aperture placed in the median line of the palliai chamber close to the base of the funnel.

The excretory organs of the cuttle-fishes are the kidneys and the ink-sac; and the integumentary sinuses in connection with the so-called “aquiferous pores” may possibly also have an excretory function. The kidneys (r, r, fig. 2) are spongy, cellular, tufted, or massive organs appended to the two posterior branches of the vena cava, and sometimes developed on others of the principal veins, just before they open into the branchial hearts. They are contained, along with the veins to which they are attached and the corresponding branchial heart on each side, in two serous sacs, which are separated centrally by the chamber containing the systemic heart, and which open by distinct apertures into the pallial chamber. The renal appendices are in direct communication with the veins on which they are situated, and have the form of membranous, often plicated sacs, covered externally with a layer of glandular cells which secrete a yellowish fluid. This fluid escapes into the serous sacs surrounding the kidneys, and is thence expelled into the mantle-cavity by the apertures before mentioned. The identity of this fluid with the renal secretion of the higher animals is shown by its containing uric acid (as proved by Harless).

The ink-sac is a glandular organ, present in all known Dibranchiates, generally of a pyriform shape, situated in different portions of the visceral chamber, but communicating by a longer or shorter duct either with the terminal portion of the intestine, or, more commonly, directly with the pallial cavity by a special aperture of its own, situated close beside the anus and at the base of the funnel. The ink-sac has strong fibrous walls, often with a silvery lustre, and its secretion is a brown or black fluid, containing a large amount of a carbonaceous pigment (“sepia ”), along with various mineral salts. It is employed by the cuttie fishes as a means of protection against their enemies, as they have the power of at will expelling jets of it into the surrounding water, and thus raising a cloud under cover of which they make their escape. The colouring matter of the ink is highly indestructible, is often found preserved in fossil Dibranchiates, and was formerly employed in the manufacture of the paint “sepia.”

The circulatory organs of the cuttle-fishes consist of arteries, veins, generally an intermediate system of capillaries, a more or less extensively developed system of sinuses or lacunae amongst the tissues, a central systemic heart, and two accessory or “branchial” hearts, whereby the venous blood is propelled through the breathing organs. The general course of the circulation is as follows. The venous blood returned from the arms, the anterior portion of the body and mantle generally, and the funnel is collected into a main ventrally-placed vein (the vena cava). This is reinforced by a great vein (canalis venosus), which brings the blood from a venous sinus surrounding the buccal chamber, the gullet, stomach, and liver in the Octopods, but which is of comparatively small dimensions in the Decapods. The great venous channel thus formed splits into two branches (the so-called “branchial arteries”), which further receive the venous blood returned from the posterior viscera and hinder portion of the mantle by special vessels (the “visceral veins ”). The branchial arteries then pour their contents by valvular apertures into two special muscular contractile chambers, which are termed the “branchial hearts,” and are situated, one on each side, at the bases of the gills. The branchial hearts drive the blood through the gills, where it is aerated, and whence it issues, as arterial blood, by the so-called “branchial veins,” which convey it to the true systemic heart. This organ is placed in the middle line of the body, between the renal sinuses on each side, and below the bifurcation of the vena cava. The two branchial veins open into it by contractile dilatations, which may be regarded as auricles, and it consists of a single muscular cavity, which propels the blood into the systemic aorta. By this vessel the aerated blood is distributed to the tissues generally, finding its way back to the veins mostly through the intervention of a system of capillaries, but partly by the venous sinuses and lacunae before-mentioned. Besides the true aorta there arise from the systemic heart minor arterial vessels, by which the blood is conveyed to the mantle, fins, and reproductive organs. The blood contains microscopic corpuscles, and is remarkable in containing a notable amount of copper.

The respiratory organs of the cuttle-fishes consist of two gills or branchiae, one of each side of the body, placed in the cavity of the mantle-sac. The branchiae are of an elongated pyramidal figure, and each consists of a central stem, attached below to the visceral mass and along one side to the mantle, the other side being free. The central stem bears a larger or smaller number of triangular laminae, in turn supporting similar secondary laminae, which finally carry still smaller tertiary laminae; and the blood is thus minutely distributed through the gill. In the absence of a ciliated branchial surface, the necessary respiratory currents are maintained by the alternate contractions and expansions of the muscular walls of the pallial chamber. As the mantle dilates, the water from the exterior makes its way into the mantle-cavity by the opening between the rim of the maistle and the neck. The water, after passing over the gills, is then expelled through the funnel by the contraction of the mantle. The course of the out-going current through the funnel is determined either by the presence of suitable valves at the base of this organ, which permit the egress of water, but do not permit its ingress, or by the articulation of the sides of the funnel with the rim of the mantle-sac, in such a manner that the opening into the mantle-cavity is completely closed during the exspiratory act, the funnel alone remaining open. As before remarked, the outgoing respiratory currents serve to take with them the excrementitious portions of the food and the secretions of the kidneys and ink-sac; and they are also concerned in swimming, as the animal can by their means propel itself backwards through the water.

The nervous system of the cuttle-fishes consists of the three principal pairs of ganglia characteristic of the Lamellibranchs and Gasteropods—namely, the cephalic, pedal, and parieto-splanchnic. These form an aesqphageal collar, which consists of a smaller dorsal mass (the cephalic ganglia), and a larger ventral mass (the pedal and parieto-splanchnic ganglia), united by commissures. The central organs of the nervous system are protected by a cartilage, foreshadowing the cranium of the vertebrate animals; and the cerebral ganglia supply the nerves to the buccal mass and eyes, whilst the pedal ganglia supply nerves to the auditory organs, funnel, and arms, and the nervous supply of the mantle and viscera generally is derived from the parieto-splanchnic ganglia. The organs of sense of the cuttle-fishes are highly developed, and consist of the eyes, auditory sacs, and perhaps of an olfactory apparatus. Space will not permit of a description of these, and it must suffice to say that the organs of vision are of large size, and more highly developed than in any other invertebrate animals, consisting of a sclerotic, choroid, retina, vitreous humour, aqueous humour, and crystalline lens; the organs of hearing are two chambers hollowed out of the cartilage of the cranial plate, each containing a membranous sac with an otolith; and the organs of smell have been doubtfully sought in certain cavities which open by small apertures behind the eye, and are supplied with filaments from the cephalic ganglia.

Of the skeletal structures which these animals possess, some are integumentary and exoskeletal, whilst others may be regarded as constituting a true endoskeleton. In the latter category are the various internal cartilaginous structures which are found in the cuttle-fishes, protecting vital organs, or serving as a base of insertion for muscles. Some of these, such as the cartilages which strengthen the anterior rim of the mantle-sac, and serve for its articulation in some forms with the funnel, and the cartilages of the fins (when present) need no further notice; but the cranial cartilage is of greater importance. This latter forms a plate, which surrounds the gullet, and incloses and protects the great oesophageal nerve-collar, and which sends off prolongations which strengthen and protect the eye, thus discharging the functions of the orbits of higher animals.

The integumentary skeleton of the cuttle-fishes consists of an external shell only in the Paper Nautilus (Argonauta), but in all others in which it is présentât all it is composed of certain horny or calcareous structures, which are lodged in the substance of the mantle, and are therefore concealed from view. This internal skeleton, so characteristic of the Dibranchiates, is well developed in all the Decapods, but is either absent or rudimentary in the Octopods, in which it never consists of more than “two short rudimental styles encysted in the dorso-lateral parts of the mantle” (Owen). In the typical Decapods (such as Loligo, Sepioteuthis, Enoploteuthis, Histioteuthis, &c.), the skeleton is horny, and consists of a feather-shaped “pen” (b, fig. 3) composed of a central shaft and two more or less extensively developed lateral expansions or wings, the whole imbedded in the mantle in the middle line of the back. In some cases (Onychoteuthis, Ommastrephes, Loligopsif) the hinder end of the pen is developed into hollow conical appendix or cup, forming a rudimentary “splanchnoskeleton.” In the genus Sepia, the internal skeleton (a, fig. 3) consists of a horny oval plate, strengthened by calcareous matter, which is deposited principally on its internal surface, and consists of numerous thin plates separated by vertical fibres. The “cuttlebone,” or “sepiostaire,” is of a porous, spongy consistence, and is concave on its inner surface behind, terminating posteriorly in a small cone (“mucro ”), from which...