“The _octopus_, which was the animal denominated polypus by Aristotle, has eight arms of equal length, and contains in its interior two very small rudimentary shells, formed by the inner surface of the mantle. This shell becomes much more distinct in the loligo, where it is cartilaginous, and shaped like the blade of a sword. The internal shell of the common _sepia_ is large and broad, and composed wholly of the carbonate of lime; it is well known by the name of cuttle-fish bone. Its structure is extremely curious, and deserves particular attention, as establishing the universality of the principle which regulates the formation of shells, whether external or internal, and from which structures differing much in their outward appearance may result. It is composed of an immense number of thin calcareous plates, arranged parallel to one another, and connected by thousands of minute hollow pillars of the same calcareous material, passing perpendicularly between the adjacent surfaces. This shell is not adherent to any internal part of the animal which has produced it, but is inclosed in a capsule, and appears like a foreign body impacted in the midst of organs with which, at first sight, it appears to have no relation. It no doubt is of use in giving mechanical support to the soft substance of the body, and especially to the surrounding muscular flesh; and this probably contributes to the high energy which the animal displays in all its movements. It has been regarded as an internal skeleton, but it certainly has no pretensions to such a designation; for, although enveloped by the mantle, it is still formed by that organ, and the material of which it is composed still carbonate of lime. On both these accounts it must be considered as a true shell, and classed among the productions of the integuments. It differs, indeed, from bony structures, which are composed of a different kind of material, and formed on principles of growth totally dissimilar. Besides tentacula, the _sepia_ is also provided with a pair of fleshy fins, extending along the two sides of the body. The _loligo_ has similar organs of a smaller size, and situated only at the extremity of the body which is opposite to the head. They have been regarded as the rudiments of true fins, which are organs developed in fishes, and which are supported by slender bones; but no structure of this kind exists in the fins of the _Cephalopoda_. In swimming, the organs principally employed by cuttle-fish for giving an effective impulse to the water are the tentacula. These they employ as oars, striking with them from behind forward, so that their effort is to propel the hinder part of the body, which is thus made to advance foremost, the head following in the rear. They also use these organs as feet for moving along the bottom of the sea. In their progress under these circumstances, the head is always turned downward and the body upward, so that the animal may be considered as literally walking on