The members of this order of the Cephalopoda are characterised by being creeping animals, protected by an external, many-chambered shell, the septa between the chambers of which are perforated by a membranous or calcareous tube termed the " siphuncle." The arms are numerous and are devoid of suckers ; the branchiae are four in number, two on each side of the body; the funnel does not form a complete tube; and there is no ink-bag.

Though abundantly represented by many and varied extinct forms, the only living member of the Tetrabranchiata is the Pearly Nautilus, which has been long known by its beautiful chambered shell, but the soft parts of which were first described from a perfect specimen which was examined by Professor Owen.*

Fie. 232

Fie. 232. - Diagram of Belemnite (after Professor Phillips). r Horny pen or "pro-ostracum;" p Chambered "phragmacone" in its cavity (a) or "alveolus;" g " Guard."

* The animal of the Pearly Nautilus is still one of the greatest rarities in museums. Its anatomy was originally described from a female specimen cups or suckers. Four of the arms of the male are specially modified to form a peculiar organ termed the " spadix," which is connected with reproduction, and corresponds with the "hectocotylised " arm of the male Cuttle-fishes. In the centre of the head is the mouth, surrounded by a circular fleshy lip, external to which is a series of labial processes. The mouth opens into a buccal cavity, armed with two horny mandibles, partially calcified towards their extremities, and shaped like the beak of a parrot, except that the under mandible is the longest. There is also a "tongue," which is fleshy and sentient in front, but is armed with recurved teeth behind. The gullet opens into a large crop, which in turn conducts to a gizzard, and the intestine terminates at the base of the funnel. On each side of the crop is a well-developed liver.

The soft structures in the Pearly Nautilus may be divided into a posterior, soft, membranous mass (metasoma), containing the viscera, and an anterior muscular division, comprising the head (prosoma); the whole being contained in the capacious outermost chamber (the body-chamber) of the shell, from which the head can be protruded at will. The shell itself (fig. 233) is involuted and many-chambered, the animal being contained successively in each chamber, and retiring from it as its size becomes sufficiently great to necessitate the acquisition of more room. Each chamber, as the animal retires from it, is walled off by a curved, nacreous septum; the communication between the chambers being still kept up by a membranous tube or siphuncle, which opens at one extremity into the pericardium, and is continued through the entire length of the shell. The position of the siphuncle is in the centre of each septum, but the siphuncle simply passes through the chambers, without opening into them.

Fig. 233.   Pearly Nautilus (Nautilius pompilius). a Mantle ; b Its dorsal fold ; c Hood ; o Eye; t Tentacles; f Funnel.

Fig. 233. - Pearly Nautilus (Nautilius pompilius). a Mantle ; b Its dorsal fold ; c Hood ; o Eye; t Tentacles; f Funnel.

Posteriorly the mantle of the Nautilus is very thin, but it is much thicker in front, and forms a thick fold or collar surrounding the head and its appendages. From the sides of the head spring a great number of muscular prehensile processes or "arms," which are annulated, but are not provided with by Prof. Owen in 1832. Since that time examples have been described by Van der Hoeven, Vrolik, Valenciennes, Macdonald, etc.

The heart is contained in a large cavity, divided into several chambers, and termed the "pericardium" (Owen). The respiratory organs are in the form of four pyramidal branchiae, two on each side.

The chief masses of the nervous system are the cerebral and infra-oesophageal ganglia, which are partially protected by a cartilaginous plate, which is to be regarded as a rudimentary cranium, and which sends out processes for the attachment of muscles. The organs of sense are two large eyes, attached by short stalks to the sides of the head, two spheroidal ear-capsules, and two hollow plicated subocular processes, believed to be possibly olfactory in their function.

The reproductive organs of the female consist of an ovary, oviduct, and accessory nidamental gland.

There is no ink-bag, and the funnel does not form a complete tube, but consists of two muscular lobes, which are simply in apposition. It is the organ by which swimming is effected, the animal being propelled through the water by means of the reaction produced by the successive jets emitted from the funnel. The function of the chambers of the shell appears to be that of reducing the specific gravity of the animal to near that of the surrounding water, since they are most probably filled with some gas secreted by the animal. Good authorities, however, believe that the chambers of the shell are filled with water. The function of the siphuncle is unknown, except in so far as it doubtless serves to maintain the vitality of the shell.