RESPIRATORY ORGANS.—The branchi pre sent the same general form and structure in both orders of Cephalopods, but differ, as before ob served, in number, and also in their mode of attachment to the mantle. They are always entirely concealed and protected by the mantle, -which is extended forwards so as to form a peculiar chamber for them anterior to the other viscem, and into which the rectum and gene rative organs open. It is interesting to perceive the respiratory cavity retaining, in the highest organized Mollusks, that relation with the anal extremity of the digestive canal which we trace through the whole of this type of animal con formation, and which forms so well-marked a line of distinction between the Molluscous and Vertebrate divisions of the animal kingdom.
In the Nautilus the four branchi are at tached by their bases only to the inner surface of the mantle ; but in the Dibranchiates a thin fibrous membrane connects the fleshy stem of each gill to the contiguous surface of the man tle. In the Nautilus the branchi are subject to contortions from the want of this support ; and in the specimen which we dissected, we found the gills on one side closely bent upon themselves, with their apices turned down; this circumstance does not probably impede a cir culation which flows, with an equable and con tinuous current through the gill ; but where the blood is driven in jerks by the contractions of a powerful ventricle, a necessity then exists for the provision of a free channel for the passage of the fluid ; and accordingly we find that the obstruction of the bmuchial artery by the bending of the fleshy stem of the gill is obvia ted by the simple but effectual means above described, viz. the superaddition of a connect ing membrane, which always preserves the gill in a straight position.
In both orders of Cephalopoda the branchi present an elongated pyramidal figure, with their apices directed forwards : they are compressed from before backwards in the Nautilus (n, ?n, fig. 224), and from side to side in the Cuttle-fish (i, k, fig. 225) and most other Dibranchiates. They are composed of a number of triangular vascular laminm extendingtransversely from each side of a central fleshy stem (h,fig. 225), having an alternate disposition : each lamina is com posed of smaller transverse laminm, which are again similarly subdivided ; the entire gill thus exhibiting the structure called by botanists tri pinnate,' by vvhich an extensive surface is afford ed for the minute division of the branchial vessels.
In the Nautilus (fig. 224) there is a larger and smaller branchia on each side; the larger and external branchia (m) presents forty-eight pairs of laminze; the smaller branchia (n) thirty-six.
In the Dibranchiates the gills vary in the relative size and number of laminae in different genera ; they are, perhaps, proportionally small est in the Loligopsis, where, according to Rathke, the number of branchial laminm does not exceed twenty-four pairs ; and it is inte resting to observe in this genus that the mus cular structure of the mantle has a correspond ingly feeble development. In the Cuttle-fish
the branchim are each composed of thirty-six pairs of triangular laminm : in the Sagittated Calamary of sixty pairs of laminaa.
As the branchiw of the Cephalopods are un provided with vibratile cilia, respiration is effected by the alternate dilatation and contrac tion of the branchial chamber ; in the first ac tion the sea-water rushes in by the anterior aper ture of the mantle ; by the second it is expelled through the cavity of the funnel. As in other classes, respiration is performed more quickly in the young than in the full-grown animals : Dr. Coldstream witnessed an Eledone, which measured one inch and a half in length, respire eighteen times in a minute; while one of the same species, which measured four inches in length, respired ten times in a minute. The proper direction of the respiratory currents is insured by various mechanical contrivances ; in the Nautilus, the funnel passes through a hole in the,substance of the mantle, which fits it so closely, that at the moment when the funnel is distended by the expiratory stream, no space is left external to it by which the water can escape ; and the greater the force by which the water is driven into the funnel, the closer is it girt by the mantle. In the Poulp and Eledone, where the funnel is connected to the fore part of the neck, and the mantle passes across its base, two large valvular folds (one of which is shown at v,fig. 216) are extended from its sides; these are concave towards the respiratory sac; they subside during inspiration, and the parietes of the funnel at the same time are collapsed ; the latter during expiration are dilated, while the valves are raised and expanded, and thereby prevent the ejected currents from passing out side the funnel. In the Argonaut, and in all the Decapods, except the Loligopsis and Cranchia, the sides of the funnel are articula ted to the opposite sides of the mantle by ball and-socket joints, which produce so close an apposition of the anterior free margin of the mantle with the parts it surrounds, that upon its contraction, no other outlet, save the funnel, is left for the expiratory currents. Iri the Ar gonaut the pallial eminence is a round tuber cle, below which is a small cavity, and these are adapted to a cavity and tubercle of corre sponding form at the side of the funnel. In Sepia, the articular tubercle is elongated in the direction of the axis of the body, and is of an oval form. In Loligo and Onychoteuthis it is still more elongated and narrow, and the arti cular depression is conformable : in Loligopsis the corresponding cartilage is no longer sub servient to an articulation with the funnel, but is represented by a series of wait-like knobs.