Simple or compound, free or sessile, peopling the high seas or lining the shores, the marine Molluscs, branching out into more than ten thousand species, extend their reign as far as the waves of ocean roll. Though distinguished from all other sea-animals by the common character of a soft unarticulated body, possessing a complicated digestive apparatus, and covered by a flexible skin or mantle, under or over which a calcareous shell is generally formed by secretion, yet their habits are as various as their forms. Some dart rapidly through the waters, others creep slowly along, or are firmly bound to the rock; in some the senses are as highly developed as in the fishes, in others they are confined to the narrow perceptions of the polyp. Many are individually so small as to escape the naked eye, others of a size so formidable as to entitle them to rank among the giants of the sea; some are perfectly harmless and unarmed, others fully equipped for active warfare. It is evident that creatures so variously gifted, and consequently so widely dissimilar in structure, cannot possibly be grouped together in one description, and that each of the four orders, Cephalopoda, Gasteropoda, Pteropoda, and Acephala (Lamellibranchiates, Brachiopods, Polyzoa, and Tunicata), into which they have been subdivided, must be separately brought before the reader, in order to give him a clear and faithful picture of their organisation and mode of life.
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.
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.