Page:Popular Science Monthly Volume 21.djvu/775

Rh stalks in the ten-armed cuttle-fishes, but are unstalked in the eight-armed species. Each sucker (Fig. 2) exhibits all the structures incidental to an apparatus adapted to secure effective and instantaneous adhesion to any surface. It consists of a horny or cartilaginous cup (a), within which are muscular fibers converging toward its center, where they form a well-defined plug or piston (b). By the withdrawal of this plug a partial vacuum is produced, and the suckers adhere by atmospheric pressure to the surface on which they are placed. The sucker is released by the projection of the plug and by the consequent destruction of the vacuum. The number of the suckers varies, but is always considerable; and when we reflect that the array of suckers can be instantaneously applied, and that their hold is automatically perfect, the grasp of the cephalopods is seen to be of the most efficient kind. In some cuttle-fishes, and most notably in the so-called "hooked squids" (Onychoteuthis), the pistons of the suckers are developed to form powerful hooks, by means of which the prey may be secured with additional facility; and in the common squids the margin of the sucker is provided with a series of minute horny hooks. The "arms" themselves, it need hardly be remarked, are extremely mobile; they are highly muscular, and can be adapted with ease to the varied functions of prehension and movement they are destined to subserve. As regards their arrangement, they are arranged in four pairs—a dorsal and a ventral pair, and two lateral pairs; the two elongated tentacles, when developed, being situated between the third and fourth pairs of arms on the ventral or lower surface.

The alimentary tract or digestive system of the cuttle-fish race is in every respect of well-developed and complete character. Lower down in the molluscan series the commissariat department is subserved by a very perfect digestive apparatus, including representatives of most of the organs familiar enough to us in higher or vertebrate existence. In the cephalopods we should naturally expect the standard of lower-molluscan organization to be further elaborated; and this anatomical expectation is justified by the actual details of cuttle-fish structure. The mouth opens on the upper surface of the head—a disposition of matters already accounted for when considering the relations of the cuttle-fish body to that of other mollusks. The mouth-opening is usually bounded by a raised lip, and leads into a cavity containing an elaborate apparatus, analogous to the jaws of higher animals, and by means of which the food of these animals is triturated and divided. An inspection of the masticating apparatus of a cuttle-fish readily solves the question, "How are the hard shells of their