There are two principal groups of Cephalopods, viz., the Decapods and the Octopods. The Decapods, as their name indicates, have ten feet or arms surrounding the mouth, and in them the body is usually elongate, containing a horny "pen" or calcareous "bone." This group includes the cuttlefishes or sepias, from which are obtained sepia ink and the cuttlefish bone used to feed canary birds. The ink is a secretion which the cuttlefish discharges when attacked to create a cloud in the water and thus escape unperceived. The squids (Loligo) commonly used as bait by fishermen belong to the Decapoda. The two extra feet or arms which the Decapods have in addition to the eight possessed by the Octopods, differ from the others in being longer and slenderer and having suckers only on the distal extremities which are expanded into "clubs" (fig. [110]).

Fig. 110.—The giant squid, Ommatostrephes californica. (From specimen with body (exclusive of tentacles) four feet long, thrown by waves on shore of the Bay of Monterey, California.)

The Octopods have a short, sac-like, sub-spherical body and neither external nor internal shell. To this group belong the famous devil-fishes (Octopus), whose strange and terrifying appearance combined with their frequently great size has furnished the basis for many a weird tale of the sea. Octopi have been killed having tentacles more than 30 feet in length. The largest members of the class, however, are probably the giant squids (belonging to the Decapoda) specimens of which have been captured with a body-length of twenty feet, and arms thirty-five feet long.

The beautiful paper sailor or argonaut (Argonauta argo), which secretes a thin shell (not homologous with the shell of the other molluscs) to protect her eggs, is a member of the Octopod group. In fine weather the argonauts sail in fleets on the surface of the ocean.

The pearly nautilus (Nautilus pompilius) is a Cephalopod with four gills instead of two, as with the Decapoda and Octopoda, and is the only existing member of what was in the earlier times of the earth's history a large group of animals. The nautili live in rather shallow water usually creeping over the bottom feeding on small marine animals. They make a many-chambered spiral shell with its inner surface lined with beautiful pearly nacre.

[CHAPTER XXIII]