CUTTLEFISH 737 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 con sists 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 lacunne 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 branchia?, one of each side of the body, placed in the cavity of the mantle-sac. The branchiae are of an elong ated 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 lamina?, in turn supporting similar secondary lamina?, which finally carry still smaller tertiary lamina? ; 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 mautle 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 Lamelli- branchs and Gasteropods namely, the cephalic, pedal, and parieto-splanchnic. These form an oesqphageal 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-splauchnic 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 taat the organs of vision are of large size, and more highly developed than in any other inverte brate 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
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