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homologous with the true shell in other cases; it is a structure sui generis secreted by the expanded arms of the dorsal pair which are closely applied to it on either side (fig. 22).

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Fig. 23.—Head and circumoral processes of the fore-foot of Onychoteuthis (from Owen).		Fig. 24.—Male of Ocythoe catenulata, Steenstrup (Octopus carena, Ver.), showing the hectocotylized arm. (From Gegenbaur.)
a, Neck. ⁠b, Eye. c, The eight short arms. d, Long prehensile arms, the clavate extremities of which are provided with suckers at e, and with a double row of hooks beyond at f. The temporary conjunction of the arms by means of the suckers enables them to act in combination.		t1, t2, t3, t4, The first, second, third and fourth arms or processes of the fore-foot. h, The third arm of the right side hectocotylized. x, The apical sac of the hectocotylized arm. y, The filament which issues from the sac when development is complete. i, The siphon.

Head, Foot, Mantle and Mantle-cavity.—If we now compare the fore-foot of the Dibranchiata with that of Nautilus, we find in the first place a more simple arrangement of its lobes, which are either four or five pairs of tapering processes (called “arms”), arranged in a series around the buccal cone, and a substitution of suckers for tentacles on the surface of these lobes (figs. 15 and 24). The most dorsally placed pair of arms, corresponding to the two sides of the hood of Nautilus, are in reality the most anterior, and are termed the first pair. In the Octopoda there are four pairs of these arms (fig. 38), in the Decapoda five pairs, of which the fourth is greatly elongated (figs. 15, 16). In Sepia, Sepiola and Rossia, each of these long arms is withdrawn into a pouch beside the head, and is only ejected for the purpose of prehension. In Loligo they are completely retractile, very slightly so in the majority of the Oigopsida, and in Rhynchoteuthis they are united to form a beak-like appendage. A gradual reduction of the tentacular arms can be seen in the Decapoda, leading to their total absence in Octopoda; thus in Leachia, Chaunoteuthis and others these arms are reduced to mere stumps. In some Cheiroteuthidae and Cranchiidae the ordinary or sessile arms, especially the dorsal pairs, are reduced. In the Octopoda they are not unfrequently connected by a web, and form an efficient swimming-bell, e.g. in Cirrhoteuthidae and Amphuretidae. The suckers are placed on the adoral surface of the arms, and may be in one, two or four rows, and very numerous. In place of suckers in some genera, e.g. Veranya, we find on certain arms or parts of the arms horny hooks; in other cases a hook rises from the centre of each sucker. The hooks on the long arms of Onychoteuthis are drawn in fig. 23. In various species of Cheiroteuthis the suckers on the tentacular arms are very feeble, but the bottom of the cup is covered by a number of anastomosed epithelial filaments which are used as a fishing-net. The fore-foot, with its apparatus of suckers and hooks, is in the Dibranchiata essentially a prehensile apparatus, though the whole series of arms in the Octopoda serve as swimming organs, and in many (e.g. the common octopus or poulp) the sucker-bearing surface is used as a crawling organ.

Fig. 25.—View of the postero-ventral surface of a male Sepia, obtained by cutting longitudinally the firm mantle-skirt and drawing the divided halves apart. This figure is strictly comparable with fig. 4. (From Gegenbaur.)
C, The head. J, The mid-foot or siphon, which has been cut open so as to display the valve i. R, The glandular tissue of the left nephridium or renal-sac, which has been cut open (see fig. 29). P, P, The lateral fins of the mantle-skirt. Br, The single pair of branchiae (ctenidia). a, The anus—immediately below it is the opening of the ink-bag. c, Cartilaginous socket in the siphon to receive c′, the cartilaginous knob of the mantle-skirt—the two constituting the “pallial hinge apparatus” characteristic of Decapoda, not found in Octopoda.	g, The azygos genital papilla and aperture. ′i, Valve of the siphon (possibly the rudimentary hind-foot). m, Muscular band connected with the fore-foot and mid-foot (siphon) and identical with the muscular mass k in fig. 3. r, Renal papillae, carrying the apertures of the nephridia. v.br, Branchial efferent blood-vessel. v.br′, Bulbous enlargements of the branchial blood-vessels (see figs 28, 29). t, Ink-bag.

In the males of the Dibranchiata one of the arms is more or less modified in connexion with the reproductive function, and is called the “hectocotylized arm.” This name is derived from the condition assumed by the arm in those cases in which its modification is carried out to the greatest extent. These cases are those of the Octopods Argonauta argo and Ocythoe catenulata (fig. 24). In the males of these the third arm (on the left side in Argonauta, on the right side in Ocythoe) is found before the breeding season to be represented by a globular sac of integument. This sac bursts, and from it issues an arm larger than its neighbours, having a small sac at its extremity in Ocythoe (fig. 24. x), from which subsequently a long filament issues. Before copulation the male charges this arm with the spermatophores or packets of spermatozoa removed from its generative orifice beneath the mantle-skirt, and during coitus the arm becomes detached and is left adhering to the female by means of its suckers. A new arm is formed at the cicatrix before the next breeding season. The female, being much larger than the male, swims away with the detached arm lodged beneath her mantle-skirt. There, in a way which is not understood, the fertilization of the eggs