Cephalopoda

Cephalopoda (or head-footed), a class of Mollusca. in which the foot is produced into three pairs of lobes; one pair of these meet in front of the mouth, and thus the head appears to be surrounded by the feet. The main characters of the class which distinguish it from other head-bearing Mollusca are these: - The body is bilaterally symmetrical; the lobes of the feet bear either tentacles or suckers, many in number; there is a powerful muscular fold on the ventral side of the visceral sac, which serves both for respiration and locomotion; the possession of an elongated visceral mass; a highly developed blood-vascular system, with a pair of auricles and branchial hearts; and a series of pigment spots in the skin, which the animal is able to contract or expand, and so change its colour into a resemblance to that of surrounding medium. Of these points the most important are those relating to the development of the foot, which is composed of three regions, each formed by the union of a pair of lateral lobes. The three regions are the propodium, mesopodium, and metapodium. Of these the first is the anterior; it is formed by the two lobes growing round in front of the head; the margin of the propoclium is continued out into a series of eight or ten arms provided with suckers (Octopus, Squid, etc.), or of the same number of lobes bearing numerous tentacles (Nautilus). The mesopodium is a flat fold of muscle on the ventral side; it forms a chamber opening to the exterior by a pair of flaps (Nautilus), which have fused into a funnel in the Cuttle Fish, Squid, etc. The hind foot, or metapodium, forms a small valve by which the siphon may be closed. The use of the chamber is partly respiratory and partly locomotor. The chamber may be filled with water, which, on the contraction of the muscular fold, is driven out of the narrow aperture and thus the animal is propelled backwards.

The shell of the Cephalopoda is very characteristic, and very variable in form and importance. In the two orders of the Nautilus and its allies (Nautiloidea) and the Ammonites and their allies (Ammonoidea), the shell was external; it was a hollow cone, straight, curved or coiled, divided into a series of chambers by transverse septa; the whole of the chambers were connected by a membranous tube (siphuncle), which was on the outer curve of the shell in the Ammonites, but central or internal in the Nautili. In the Ammonoidea the margins of -the septa (suture lines) are usually very sinuous, while in the Nautiloidea they are simply curved. The structure of the young shell is, moreover, very different in these two orders; in the Ammonoidea the embryonic shell (protoconch) is preserved; in the Nautiloidea it is lost, and its place is marked by a scar. In the third order the shell has become enclosed by the mantle; it is coiled and chambered as in Spirula, or conical and chambered as in Belemnites and Sepia. A chitinous sheath, deposited by the enveloping mantle, forms the "guard" of Belemnites, the "mucro" and shagreen layer of Sepia. In some forms, e.g. the Squid, this sheath is all that remains of the shell, and is called the "gladius" or "pen." In Octopods even this also is lost.

The respiratory organs consist of one or two pairs of gills or ctenidia, placed in the branchial chamber; they are aerated by water taken into this chamber and forced out through the siphon. The excretory organs (nephridia) lie at the base of the gills, and are, like them, in one or two pairs. Among special organs in the Cephalopoda may be mentioned the ink sac of the order with enclosed shell. This is a bag close to the stomach, with a duct passing out by the anus to the funnel. The sac is filled with sepia, some of which is discharged when the animal wishes to conceal itself. As the shell of the "Squid" consists of a long narrow quill-like plate.s the animal has consequently acquired the name of the "pen-and-ink fish."

The Cephalopoda were formerly classified as Tetrabranchiata and Dibranchiata. The former included the more primitive forms with four gills, four nephridia, a primitive "penhole" eye, an external chambered shell, traversed by a membranous tube, the "siphuncle"; they were represented by only one living genus, Nautilus. Modern writers separate the forms with external shell into the two orders Nautiloidea and Ammonoidea. As to the internal organs of the latter order very little is known. The third order with enclosed shell practically corresponds to the old group Dibranchiata. The members of this order have but two gills and nephridia, large and specialised eyes, a funnel, ink-bag, and arms bearing suckers. They are divided into the sub-orders (i) Decapoda, with ten arms and pedunculate suckers, as in the Squid; (ii) Octopoda, with eight arms and sessile suckers, ees the Argonaut and Devil-fish.

The oldest Cephalopod comes from the Tremadoe rocks in the Cambrian system.