Mollusca in which the head region is more or less developed, and is always provided with an odontophoral apparatus and radula.
Bilaterally symmetrical Glossophora, with the head surrounded by a portion of the foot which is produced into tentacle-bearing lobes or sucker -(sometimes hook-) bearing arms; with a part of the foot modified to form a siphon; and a muscular contractile mantle-fold subserving both respiration and locomotion. The shell is variable in structure, sometimes internal or absent; when external usually chambered. The visceral dome is elongated. The ctenidia, which are usually two in number, the anus and nephridial apertures, are posterior. The vascular system is highly developed, and consists of a ventricle and usually two auricles, with a pair of branchial hearts. The pericardium is large, the nephridia saccular. The sexes are separate, and the sperm is enveloped in a spermatophore. Chromatophores are remarkably well developed in the integument.
The fore-foot has grown round the head, and its margins are produced into lobes or arms; of the arms there are eight in Octopoda, ten in Decapoda, arranged in pairs. The four pairs of arms in Octopoda are similar, and are sometimes connected laterally to one another by a thin membrane which may extend almost to their tips. The fourth pair (reckoning the median dorsal pair as the first) is modified in the Decapoda into a pair of long extensile arms which can be partially or completely (e. g. Sepia) retracted into sacs at their bases. The arms are provided on their adoral aspect, or at their expanded extremities in the case of the long arms of Decapoda, with suckers or acetabula arranged in 1-4 rows, or with hooks. The suckers of the Decapoda are stalked, and the cup has a marginal horny ring armed in some instances with pointed teeth. They are sessile and devoid of a ring in Octopoda. In Nautilus (Tetrabranchiata), the fore-foot is divisible into an outer and inner portion. The outer portion surrounds the whole head, and is thickened dorsally where it abuts against the coil of the shell and forms the hood1. It carries nineteen tentacles on each side, of which two are median and dorsal.
The inner portion forms three distinct lobes in the female, i. e. a right and left lateral lobe, present also in the male, and an inferior or ventral lobe, represented in the male by three groups of lamellae. The lateral lobes each carry twelve tentacles, and are divided in the male into two portions, a more dorsal with eight tentacles, and a ventral which carries four tentacles on the right side, and is known as anti-spadix, but on the left is represented by the spadix, a conical body terminated by imbricated lamellae, and supposed to have a sexual function like the hectocotylised arm in Dibranchiata (p. 464). The tentacles are long solid cylindrical bodies, retractile into muscular sheaths. The sheath with its tentacle probably corresponds to a single sucker of the Dibranchiate arm. The median portion of the foot forms the siphon, the base of which is covered by the mantle fold. It consists in Nautilus of a right and left lobe with the free edges simply apposed, but in all Dibranchiata grown together so as to form a tube.
The hind part of the foot is perhaps represented by the valve of the siphon, absent in Octopoda, which projects from the body-wall within the siphon.
1 The fossils known as Aptychus, Andptychus, Synaptychus, and Trigonellites, which are sometimes found within the aperture of the shell in certain Ammonoidea, or just outside it, or entirely dissociated from it, have at various times received different explanations. Of these the two which are probable are the following: (1) that they were secreted by the surface of a hood corresponding
The body or visceral dome is elongated in an oblique direction between the dorso-pedal and antero-posterior axis. It is never twisted spirally. Its sides are expanded into a pair of variously shaped fins in Decapoda which are only exceptionally present in Octopoda (Pinnoctopus, Cirrhoteuthis). The mantle forms a free fold round the edge of the visceral dome. The dorsal margin of the fold in Nautilus is enlarged and . reflected over the shell: it is very shallow in Dibranchiata. The ventral or posterior part of the fold is of great extent in all Cephalopoda and incloses a branchial cavity.
There is no shell in the Octopoda except in the female Argonauta, but a rudimentary shell-sac appears on the anterior dorsal aspect of the visceral dome and then aborts. The external calcareous shell of Argonauta is formed by the expanded ends of the pair of dorsal arms. It lodges the animal which is not fixed to it, is single-chambered and coiled. The shell-sac of Decapoda gives origin to a shell which does not lodge the animal; it either remains open as in Spinda, or closes so that the shell is internal, in other living forms, as well as in the extinct Belemnitidae and perhaps in the extinct Spirulirostra. The shell of Spirula is coiled, chambered, and the chambers are connected by a siphuncle (infra). That of Spirulirostra is very similar; of the Belemnitidae straight. But in these extinct forms calcareous lamellae are superadded to the chambered shell. The latter is known in Belemnitidae as 'phragmacone:' the lamellae as 'guard' or 'rostrum,' and their prolongation forwards, whether to the hood of Nautilus; (2) that they are the homologues of the nuchal cartilages of Dibranchiata, a view based on their microscopic structure. See Von Ihering, Neues Jahrbuch f.
Mineral. Geol. and Palaeont. 1881, i. horny or calcified, as 'pro-ostracum.' The chambered shell is not developed in any living Decapoda except Spirilla. The part homologous with the guard is alone present. It constitutes the sepiostaire, or cuttle-bone of Sepia, composed of calcareous lamellae inclosing air spaces, and the horny pen, or gladius of Loligo, composed of conchiolin.