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. [Illustration: Poulp (Octopus).] 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. [Illustration: 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. This widens as it descends, and contains a cone of soft substance, rising from the bottom of the cavity, like the piston of a syringe. When the sucker is applied to a surface for the purpose of adhesion, the piston, having previously been raised so as to fill the cavity, is retracted, and a vacuum produced, which may be still further increased by the retraction of the plicated central portion of the disk. So admirably are these air-pumps constructed, and so tenacious is their grasp, that, when they have once seized or fixed upon a prey, it cannot possibly disengage itself from their murderous embrace. In many of the decapods, who, generally seeking their prey in the deeper waters, have to contend with the agile, slippery, and mucus-clad fishes, more powerful organs of prehension have been superadded to the suckers. Thus, in the Calamary the base of the piston is enclosed by a horny hoop, the margin of which is developed into a series of sharp-pointed curved teeth; and in the still more formidable Onychoteuthis each hoop is produced into the form of a long, curved, and sharp-pointed claw (_f_), which the predacious mollusc presses firmly into the flesh of its struggling victim, and then withdraws by muscular contraction. [Illustration: Section of an arm and suckers of a Poulp. _e._ Soft and tumid margin of the disk. _g._ Circular aperture. ]