The definition of the Cephalopoda, or cuttle-fish class, is largely a matter of commonplace observation. Linnæus, naming them "cephalopods," or "head-footed" mollusks, indicated the structural feature which was calculated to appeal most plainly even to non-technical minds. The circlet of arms, feet, or tentacles crowning the head-extremity of a cuttle-fish, thus presents us with a personal character of unmistakable nature. It is necessary, however, to bear in mind that the ordinary and, to a certain extent, natural fashion of representing a cuttle-fish head upward is, in zoölogical eyes, a complete reversion of its surfaces. To understand clearly why to speak of a cuttle-fish head as its lower, and of its tail as its upper extremity, is a correct zoölogical designation, we must enter upon a comparison of the cuttlefish body with the forms of its neighbor mollusks. The contemplation of such a familiar being as a snail or whelk introduces us to a characteristic example of molluscan form and anatomy. The head of the snail or other gasteropod is clearly enough defined; and no less plainly discernible is the enlarged and broadened surface on which the animal walks. This surface is known as the "foot." In one shape or another this "foot" is a characteristic possession of the molluscan tribes. In a section of a mussel or cockle, we perceive the "foot" to exist as a muscular mass developed in the middle line of the body below, and variously used in the mussel class as a spinning organ, a leaping-pole, and a boring apparatus. Here we note the natural development of the foot in the middle line of the animal. Let us suppose this foot to be extended downward, and to be broadened so as to form a surface of progression, and we may conceive readily of the modification whereby a simple foot like that of the mussel becomes developed to form the enlarged disk of the gasteropod. In the latter case we observe that the foot occupies the floor of the body; the bulk of the body, and the head in particular, being borne above.
Cuttle-fish development can be shown to run, so far, in parallel lines to those of the personal evolution of mussel and snail. But divergent paths soon appear in cuttle-fish development; and these variations, while they indicate an ancient departure from the ordinary molluscan type, likewise give to the subjects of our present study their most characteristic features. When a mussel or snail is watched in its earlier stages of development, the embryo is seen, sooner or later, to produce an. appendage highly characteristic of molluscan young at large, and named the velum. By aid of this ciliated fold such an organism as a young cockle, for instance, swims freely through its native waters. This velum undergoes varied changes and alterations in the after-stages of molluscan development; but, when cuttle-fish development is studied in its fullest details, no velum is found among the possessions of the larval body. Such an omission has naturally been made the subject of remark by naturalists. Some authorities—Grenacher, for instance—have insisted upon the recognition of the