The shell of Tetrabranchiata, the living Nautilus and the extinct Nautiloidea and Ammonoidea, is external and is secreted by the mantle: but nothing is known as to the initial phase of its development. It is either straight, loosely or closely coiled, and then either in a spiral like a snail, or more generally in the same plane as in Nautilus and many extinct forms. It is chambered, and the last or largest chamber is the one occupied by the animal which is attached to it by a muscle. Successive chambers are separated from one another by septa, and the unoccupied chambers are filled with air. The external edges of the septa are simple in Nautiloidea, but in the older chambers of the Ammonoidea are thrown into folds. A forwardly projecting fold is known as a 'saddle:' one that projects backwards as a 'lobe.' The edges of the folds are generally not simple but wavy, sometimes in a highly complex manner. The chambers are traversed by a tube - the sipho or siphuncle, which has membranous walls, prolonged from the integument, protected however by an outer coating of nacre continuous with the substance of the septa. This nacreous coat or 'collar' may form a complete investment or only a partial one to the membranous siphuncle.

In the latter case it projects backwards from the septa in nearly all Nautiloidea, forwards in Ammonoidea, except sometimes in the first chambers. The siphuncle is placed centrally in most Nautiloidea, but it may be either near the concave or the convex side of the shell. The latter is its position in Ammonoidea, except occasionally in the first chambers. In Nautilus the convex side of the shell corresponds to the ventral aspect of the animal, as it appears to do in Ammonoidea; but extinct Nautiloidea vary in this respect, even within the limits of the same genus.

The first chamber of the shell or protoconch has certain special features. In Ammonoidea, as also in the chambered shell of Dibranchiata, it is enlarged as compared with the next chamber; its anterior wall is bulged inwards by the sipho which does not extend through it; it is slightly dilated and covered with nacre forming the nucleus; and is often traversed by a 'pro-sipho' not connected with the siphuncle. In the Nautiloidea it is small as compared with the next chamber and only slightly curved, not coiled as in the coiled Ammonoidea, and the siphuncle traverses it and commences at its apex, which is marked externally by a 'cicatrix.' The presence of this cicatrix has been explained on the supposition that there is a deciduous protoconch (Hyatt). The differences observable have led some authorities to associate the Ammonoidea with the Dibranchiata. The shell is often variously ornamented externally with striae, knobs, etc.; it is composed of an outer layer with round or oval calcareous bodies irregularly arranged, and coloured in Nautilus and many Nautiloidea, and an inner layer of nacre or mother of pearl which is laminated and forms the whole substance of the septa.

The ectoderm consists of a single layer of cells with striated cuticular borders. Cilia are found on the tentacles and eyes of Nautilus. The underlying connective tissue contains near its surface a layer of chromato-phores, and of cells containing iridescent rods. The former are under the control of the nervous system, the centre appearing to lie in the optic ganglia. The chromatophore consists essentially of a cell charged with pigment granules either scarlet, yellow, blue, or brown, one colour in each cell. By the expansion of the cell the pigment is diffused, by its contraction concentrated. The cell has been described as lying in a space traversed by radial fibres which are attached to the cell-membrane, and supposed by various authorities to be muscular, nervous, or composed of connective tissue. See the original authorities, infra. The arms, siphon, and the mantle fold forming the walls of the branchial cavity are muscular. The mantle is contractile, the contractions expelling the water from the branchial cavity for purposes of respiration, and when forcible subserving locomotion as well.

Masses of a peculiar cartilage are formed in the body-walls, supporting the nervous system, the siphon in Nautilus, the bases of the fins in Sepia; and in all Decapoda on the dorsal aspect of the neck as nuchal cartilages, and on the opposing points of the mantle surface as dorsal cartilages, the latter present also in Octopoda; and as 'siphon-hinge' cartilages or sockets at the sides of the siphon in Decapoda which receive fleshy processes developed on the mantle surface. Pores variable in number exist on the back of the head, or at the bases of the arms, leading into sub-integumental sacs of unknown function. A sac exists at the base of each long arm in Decapoda; and in Loligo the arm may be partially, in Sepia, Sepiola, and Rossia completely, coiled within it.

The nervous system consists of three pairs of ganglia concentrated round the oesophagus - a cerebral, a pedal, and a pleuro-visceral. They have a band-like form in Nautilus, and are not well differentiated from the connectives. The cerebral ganglia are connected with buccal ganglia in the Dibranchiata, a single pair in Octopus, a double in Decapoda. The pedal ganglia supply the arms and lobes of the fore-foot, as well as the siphon. The pleuro-visceral give off a large number of nerves in Nautilus, to the mantle, branchiae, and genitalia; in Dibranchiata right and left pleural and visceral nerves to the mantle, to the branchiae and viscera respectively. Each pleural nerve ends in a ganglion stellatum, from which nerves radiate into the mantle. The Dibranchiata also possess a well-developed enteric system of nerves connected with the buccal ganglia and visceral nerves. Nautilus alone has a right and left osphradial papilla, supplied from the visceral ganglia, between the bases of the gills on each side. A ciliated olfactory pit lies behind the eye in Dibranchiata, a triangular olfactory (?) papilla below the eye in Nautilus. The nerve in both instances originates from the spot where the pedal and optic ganglia unite.