Cuttlefish

Cuttlefish, the popular name of the Sepias or Sepiidae. The term is, however, often used more generally to the whole of the two-gilled Cephalopods, including the squids, devilfish, spirula, and paper nautilus (Argonauta). The true "cuttles," however, belong to the sub-order "Decapoda," and the section Sepiophora. They may therefore be defined as Cephalopods provided with a pair of gills (or branchiae) and an ink sac, a tubular funnel, a pair of tentacles in addition to the eight equal arms, and in which the skeleton is internal and calcareous, and has no phragmocone or only a rudimentary one. The shell or sepion, or "cuttlebone," is the most characteristic feature. This consists of a flattish oval plate composed of numerous thin plates separated by small pillars; the whole is composed of carbonate of lime. At the lower end is a small sharp point known as the mucro; this is composed of very dense tissue, and it is hollowed out in front to receive a small chambered structure which is the rudimentary phragmocone. If the cuttle-bone be compared with a Belemniti (q.v.) the phragmocone may be seen in each, while the small mucro of the former represents the "guard." and the main mass of the sepion is equivalent to the delicate "proostracum." Owing to the thinness of the plates of which the cuttle-bone is composed, it is very brittle and easily rubs down into powder; it is therefore of much use as polishing material.

In regard to the structure of the soft parts, the cuttles do not vary much from the ordinary Dibranchiate Cephalopods. The body is generally rounded behind with a short blunt head bearing the ten arms; the mouth opens in the space between the bases of the arms. A fin runs all round the margin of the body. The skin or integument is generally of a light colour, but this varies greatly by the expansion of the "chromatophores," small patches of pigment scattered all over the body. The arms are of two sizes; there are eight true arms, equal in size; these bear short suckers or "acetabula," by means of which the cuttlefish can firmly hold its prey; the remaining two arms are much longer, and are known as tentacles; they are concerned with the transmission of the male reproductive elements or "spermatophores" to the genital chamber of the female. The mouth is armed with a chitinous beak; the oesophagus is short and leads to a simple pyriform stomach at the hinder end of the body; the intestine is straight and opens to a "pallial chamber" on the ventral side of the animal; thence the excreta escapes through the funnel. The nephridia or kidneys and the ink sac also both open to the pallial chamber; the ink sac is a glandular organ which secretes a black-coloured pigment mixed with various mineral salts; by the ejection of a drop of this "sepia" the water is rendered so cloudy that the cuttle can effect its escape. The blood system consists of a large central heart from which the blood passes to two accessory or "branchial" hearts; these drive it to the gills, where it is aerated and returned to a pair of auricles and thus to the true heart. There are three main pairs of nerve ganglia; commissures from these form a collar round the oesophagus; the cerebral ganglia are enclosed in a cartilaginous capsule suggestive of a cranium.

The cuttle-fish are all marine; they live either crawling over the bottom of seas of limited depth or swim rapidly backward by the violent ejection of water from the pallial chamber through the funnel; they can swim gently forward by the movement of the lateral fin. The Sepias are now very widely distributed, though the number of genera and species is now somewhat limited. The oldest fossil forms occur in the Eocene system.