The Cephalopods are the most perfect specimens of the molluscan type, as the decapods are the first among the crustaceans. These remarkable creatures consist of two distinct parts: the trunk or body, which, in form of a sack, open to the front, encloses the branchiæ and digestive organs, and the well-developed head, provided with a pair of sharp-sighted eyes, and crowned with a number of fleshy processes, arms or feet, which encircle and more or less conceal the mouth. It is to this formation that the cephalopod owes its scientific name, for as the feet grow from the circumference of the mouth, it literally creeps upon its head.
Poulp (Octopus).
All the cephalopods are marine animals, and breathe through branchiæ or gills. These are concealed under the mantle, in a cave or hollow, which alternately expands and contracts, and communicates by two openings with the outer world. The one in form of a slit serves to receive the water; the other, which is tubular, is used for its expulsion.
According to the different number of their gills, the cephalopods are divided into two groups. The first, to which the poulp and common cuttle-fish belong, and which comprises by far the majority of living species, has only two sets of gills; while the second, which, in the present epoch, is only represented by a few species of Nautilus, has four, two on each side, according to the number of their arms or feet—for these remarkable organs serve equally well for prehension or locomotion. The first group is again subdivided into two orders, Octopods and Decapods, the former having only eight sessile feet, while the latter possess an additional pair of elongated tentacles, which serve to seize a prey that may be beyond the reach of the ordinary feet, and also to act as anchors to moor them in safety during the agitations of a stormy sea.
Both the arms and tentacles are furnished with suckers disposed along the whole extent of the inner surface of the former, but generally confined to the widened extremities of the latter, where they are closely aggregated on the inner aspect.
Calamary.
In all the octopods the suckers are soft and unarmed. Every sucker is composed of a circular adhesive disk, which has a thick fleshy circumference and bundles of muscular fibres radiating towards the circular orifice of an inner cavity.
