third left arm of the male (fig. 211) is developed in a cyst, and ultimately becomes a "hectocotylus," and is deposited by the male in the pallial chamber of the female.

In the Octopodide (or Poulpes) there are eight arms, all similar to one another, and united at the base by a web.

Fig. 214.-A, The common Calamary (Loligo vulgaris), reduced in size: a One of the ordinary arms; t One of the longer arms or "tentacles." B, Skeleton or "pen" of the same, one-fourth natural size (after Woodward). C, Side view of one of the suckers, showing the horny hooks surrounding the margin. D, View of the head from in front, showing the bases of the arms (a) and tentacles (), the mouth (m), and the funnel (f.)

short styles encysted in the substance of the mantle (Owen). The body is seldom provided with lateral fins. The third right arm of the male is primarily developed in a cyst, and ultimately becomes "hectocotylised."

SECTION B. DECAPODA.-The Cephalopods of this section. have eight arms and two additional "tentacles," which are much longer than the true arms, are retractile, and have expanded, club-shaped extremities (fig. 214). The suckers are pedunculated; the body is always provided with lateral fins, and the shell is always internal.

This section comprises the three living families of the Teuthida, Sepiada, and the Spirulida, and the extinct family of the Belemnitida.

The family of the Teuthida comprises the Calamaries or Squids (fig. 214), characterised by the possession of an elongated body with lateral fins. The shell (fig. 212, b) is internal and horny, consisting of a median shaft and of two lateral wings; it is termed the "gladius" or "pen," and in old specimens several may be found lodged in the mantle, one behind the other. In the common Calamary (Loligo) the fourth left arm of the male is metamorphosed towards its extremity to subserve reproduction.

In the family of the Sepiade the internal shell (fig. 212, a) is calcareous ("cuttle-bone" or "sepiostaire "), and is in the form of a broad plate, having an imperfectly-chambered apex. The broad laminated plate is extremely light and spongy, and the chambered apex is called the "mucro." In the living members of the family the body is provided with long lateral fins, sometimes as long and as wide as the body itself.

In the singular family of the Spirulide the internal skeleton (fig. 212, c) is in the form of a nacreous, discoidal shell, the whorls of which are not in contact with one another, and which is divided into a series of chambers by means of partitions or septa which are pierced by a ventral tube or "siphuncle." The body is provided with minute terminal fins, and the arms have six rows of small suckers. The shell of the Spirulacommonly known as the "post-horn"-is similar in structure to the shell of the Nautilus, but it is lodged in the posterior part of the body of the animal (fig. 212), and is therefore internal, whereas the shell of the latter is external. It really corresponds to the "phragmacone" of the Belemnite. Though the shell occurs in enormous numbers in certain localities, a single perfect specimen of the animal is all that has been hitherto obtained.

In the extinct family of the Belemnitida, our knowledge is chiefly confined to the hard parts. Certain specimens, how

ever, have been discovered, which show that the Belemnite had essentially the structure of a Cuttle-fish, such as the recent Calamary. The body was provided with lateral fins; the arms were eight, furnished with horny hooks, with two "tentacles;" and probably the mouth was provided with horny mandibles. An inkbag was present. The internal skeleton of a Belemnite (fig. 215) consists of a chambered cone-the "phrag macone" - the septa of which are pierced with a marginal tube or "siphuncle." In the last chamber of the phragmacone is contained the ink-bag, often in a well-preserved condition. Anteriorly the phragmacone is continued into a horny lamina or 'pen" (the "pro-ostracum of Huxley), and posteriorly it is lodg ed in a conical sheath or "alveolus," which is excavated in the substance of a nearly cylindrical, fibrous body, the "guard" (fig. 215, g) which projects backwards for a longer or shorter distance, and is the part most usually found in a fossil condition.

ORDER II. TETRABRANCHIATA.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 "siphunde." The arms are numerous, and are de void of suckers; the branchia 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

Horny pen or pro-ostracum;" Chambered phragmacone in its cavity (a) or "alveolus ; g"Guard."

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.*

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 outermost, capacious chamber (the body-chamber of the shell), from which the head can be protruded at will. The shell itself (fig. 216) is involuted and many-chambered, the animal being con

Fig. 216.-Pearly Nautilus (Nautilus pompilius). a Mantle; 6 Its dorsal fold; c Hood; o Eye; Tentacles; f Funnel.

tained 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

The animal of the Pearly Nautilus is still one of the greatest rarities in Museums. Its anatomy was originally described from a female specimen by Prof. Owen in 1832. Since that time examples have been described by Van der Hoeven, Vrolik, Valenciennes, Macdonald, &c.

septum, but the siphuncle simply passes through the chambers, without opening into them.

66

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 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 heart is contained in a large cavity, divided into seve ral chambers, and termed the "pericardium" (Owen). The respiratory organs are in the form of four pyramidal branchiæ, 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 earcapsules, 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