This Class includes the Octopus or Polypus, Cuttlefish, Squid, Spirula, the Paper and Pearly Nautilus. The body of the animal consists of a muscular sac, in the cavity of which the viscera are placed. In front of the body projects the head, which, in species belonging to the two-gilled section of the Class, is surrounded by eight or ten fleshy arms. A wide aperture below the head admits the water to the gills or branchiæ, which are situated in the interior of the sac, whilst a short tube, the so-called funnel or siphuncle, projects from the opening of the mantle—the water and various excretions being expelled through this tube, especially also an ink-like fluid, which is discharged by all Cephalopods (except _Nautilus_) when disturbed, in order to darken the water and thus escape their enemies. The centre of the head, between the base of the arms, is occupied by the mouth, which is armed with two horny or calcareous jaws, similar to the beak of a parrot (Fig. 39). The two large eyes are placed on the sides of the head. The arms or feet are more or less elongate, capable of movement in any direction, and, except in _Nautilus_, furnished on one side with numerous suckers, by means of which the animal attaches itself most securely to anything it may seize; they are employed in capturing food and in walking. Cephalopods walk in any direction head downwards, but can swim backwards only, being propelled in that direction by the water which they discharge with force through the funnel out of their branchial cavity. They are divided, according to the number of their gills (which is either two or four), into _Dibranchia_ and _Tetrabranchia_. Of the latter but one representative now exists, viz., the Pearly Nautilus, all other living Cephalopods being provided with but two gills, placed one on each side of the body within the mantle, as may be seen in the wax model of _Sepia officinalis_ (Case 207). The two-gilled section comprises forms with eight arms, as _Argonauta_ and _Octopus_, and others with ten arms, viz., the Cuttlefishes (_Sepia_) (Fig. 43), the Squids (_Loligo_, _Ommatostrephes_, _Sepiola_, _Chiroteuthis_, etc.), and _Spirula_. The “shell” of the Paper-Nautilus, or _Argonauta_, is too well known to require any description. Unlike the shells of other Mollusca, it is not attached to the animal by a special muscle, but is held on to the body by two of the arms, which are dilated and specially adapted for this purpose. Only the female Argonaut is provided with a shell, the male being shellless and a much smaller creature. The Argonaut-shell is therefore not a true shell, but simply a receptacle for the ova, serving at the same time for the protection of the parent. [Illustration: Fig 40. The Common Octopus (_Polypus vulgaris_), resting. ] [Illustration: Fig. 41. _Sepiola scandica_ (Natural size). British. ]