Fig. 19.—Digram of shell Belemnite (after Phillips). r. Horny pen or “proostracum”: A, conical cavity or “alveolus,” in which the chambered “phragmacone” (p) is contained: g, “guard,” or “rostrum.”

The next modification in the enclosed shell is the addition to it of secondary deposits of calcareous matter, by the inner surface of the shell-sac. Successive layers are deposited on the posterior part of the original shell, whether coiled or straight, and these layers form a conical mass, which may attain great thickness. A somewhat coiled shell with such a deposit is seen in Spirulirostra (fig. 17, C) of the Miocene. In the next stage of modification secondary secretion forms a long and broad projection of the dorsal lip of the aperture; this is well developed in the belemnites (fig. 19). Thus in these modified shells three parts are to be distinguished: the original septate shell, which has been called the phragmacone; the posterior conical deposit, called the rostrum or guard; and the anterior somewhat flat projection, called the proostracum. In the living Dibranchiata other than Spirula the phragmacone and rostrum have become very rudimentary. The shell of Sepia (fig. 20) consists almost entirely of the proostracum, the little ventral hollow posteriorly representing the phragmacone, and the posterior pointed projection, the rostrum. In the Oigopsida the shell is represented by a proostracum which is no longer calcified by forms a chitinous plume or gladuius, and a similar rudiment occurs in Loliginidac (fig. 21) and Sepiolidae. Lastly, in the Octopoda the shell is represented only by small chitinous rudiments to which the retractor muscles of the head and funnel are attached; these are paired in Octopus, unpaired in other cases as in Cirrhoteuthis.

The early appearance of the sac of the mantle in which the shell is enclosed has led to an erroneous identification of this sac with the primitive shell-sac or shell-gland of the Molluscan embryo. The first appearance of the shell-sac in Dibranchiata is shown in figs. 35, 36. Its formation as an open upgrowth of the centro-dorsal area, and the fact that it appears and disappears without closing in Argonauta and Octopus, was demonstrated by E. Ray Lankester.

Fig. 20.Fig. 21.
Fig. 20.—The calcareous internal shell of Sepia officinalis, the so-calledcuttle-bone, a, Lateral expansion; b, anterior cancellatedregion; c, laminated region, the laminae enclosing air.
Fig. 21.—The horny internal shell or gladius or pen of Lohgo.
Fig. 22.—The Argonaut in life. (After Lacaze-Duthiers) Tr. Float: Br.a, anterior arms: Br.p, posterior arms: V, the expanded portion of them, once called the sails; B, the beak; C, the shell; En, the Funnel.

In Argonauta (the paper nautilus) the female only possesses a shell, in which the body is contained; but this is not 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).

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 drawingthe divided halves apart. This figure is strictly comparable withfig. 4. (From Gegenbaur.)

C, The head.

J, The mid-foot or siphon,which has been cut openso as to display the valve i.

R, The glandular tissue of theleft nephridium or renal-sac,which has been cutopen (see fig. 29).

P, P, The lateral fins of themantle-skirt.

Br, The single pair of branchiae(ctenidia).

a, The anus—immediatelybelow it is the opening ofthe ink-bag.

c, Cartilaginous socket in thesiphon to receive c′, thecartilaginous knob of themantle-skirt—the two constitutingthe “pallial hingeapparatus” characteristicof Decapoda, not found inOctopoda.

g, The azygos genital papillaand aperture.

′i, Valve of the siphon (possiblythe rudimentary hind-foot)

m, Muscular band connectedwith the fore-foot andmid-foot (siphon) andidentical with the muscularmass k in fig. 3.

r, Renal papillae, carrying theapertures of the nephridia.

v.br, Branchial efferent blood-vessel.

v br′, Bulbous enlargements ofthe 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 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.