The members of this order of the Cephalopoda are characterised as being swimming animals, almost invariably naked, with never more than eight or ten arms, which are always provided with suckers. There are two branchiae, which are furnished with branchial hearts ; an ink-sac is always present; the funnel is a complete tube, and the shell is internal, or, if external, is not chambered.

The Cuttle-fishes are rapacious and active animals, swimming freely by means of the jet of water expelled from the funnel. The arms constitute powerful offensive weapons, being excessively tenacious in their hold, and being sometimes provided with a sharp claw in the centre of each sucker. They are mostly nocturnal or crepuscular animals, and they sometimes attain to a great size. They may be divided into two sections, Odopoda and Decapoda, according as they have simply eight arms, or eight arms and two additional " tentacles."

Section A. Octopoda

The Cephalopods comprised in this section are distinguished by the possession of not more than eight arms, which are provided with sessile suckers. The shell is internal and rudimentary ; in one instance only (the Argonaut) external. The body is short and bursiform, and ordinarily without fins.

This section comprises the two families of the Argonautidae, and the Oc-topodidae. In the former of these there is only the single genus Argonauta (the Paper Sailor, or the Paper Nautilus), of which the female and male differ greatly from one another. The female Argonaut (fig. 230) is protected by a thin single-chambered shell, in form symmetrical and involuted, which is secreted by the webbed extremities of the dorsal arms, but is not attached in any way to the body of the animal. It sits in its shell with the funnel turned towards the keel, and the webbed arms applied to the shell. The male Argonaut is much smaller than the female (less than an inch in length), and is not protected by any shell. The third left arm of the male (fig. 228) 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 Octopodidae (or Poulpes) there are eight arms, all similar to one another, and united at the base by a web. There is an internal rudimentary shell, represented by two 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."

Fig. 230.   Argonauta argo, the

Fig. 230. - Argonauta argo, the " Paper Nautilus," female. The animal is represented in its shell, but the webbed dorsal arms are separated from the shell, which they ordinarily embrace.

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. 231). The suckers are pedunculated; the body is always provided with lateral fins, and the shell is always internal.

Fig. 231.   A, The common Calamary (Loligo vulgaris), reduced in size : a One of the ordinary arms ; t One of the longer arms or tentacles.

Fig. 231. - 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 (t), the mouth (m), and the funnel (f).

This section comprises the three living families of the Teuthidae, Sepiadae, and the Spirulidae, and the extinct family of the Belemnitidae.

The family of the Teuthidae comprises the Calamaries or Squids (fig. 231), characterised by the possession of an elongated body with lateral fins. The shell (fig. 229, 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 Sepiadae the internal shell (fig. 229, 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 Spirulidae the internal skeleton (fig. 229, 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 lateral fins, and the arms have six rows of small suckers. The shell of the Spirula - commonly 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. 229), 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 its internal anatomy, Spirula is a true Dibran-chiate. It has the peculiar feature that the hinder end of the body forms a kind of suctorial disc, apparently employed to moor the animal to foreign bodies. The beaks are not calcified. The retractor muscles of the funnel spring from the inner surface of the last chamber of the shell (as in Nautilus); and this chamber also lodges the hinder termination of the liver (Owen).

In the extinct family of the Belemnitidae, our knowledge is chiefly confined to the hard parts. Certain specimens however, 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 ink-bag was present. The internal skeleton of a Belemnite (fig. 232) consists of a chambered cone - the "phragmacone" - 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 lodged in a conical sheath or "alveolus," which is excavated in the substance of a nearly cylindrical, fibrous body, the "guard" (fig. 232, g) which projects backwards for a longer or shorter distance, and is the part most usually found in a fossil condition.