[Illustration: 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 BRANCH CHORDATA: THE VERTEBRATES, ASCIDIANS, ETC. The branch Chordata includes all the backboned animals or vertebrates, comprising the fishes, salamanders, frogs and toads, lizards, crocodiles, turtles and snakes, birds, and all the quadrupeds or mammals, and includes also a few small unfamiliar ocean animals which do not look at all like the backboned animals, but which agree with them in possessing a peculiar structure called the notochord. This notochord consists of a series or cord of cells extending longitudinally through the body from head to tail, above the alimentary canal and below the spinal nerve-cord. In all the vertebrates excepting a few low forms, the notochord while present in the young, is replaced in the adult by a segmented bony or cartilaginous axis, the spinal or vertebral column. But in the ascidians or sea-squirts (called also tunicates) it persists throughout life. In addition to this characteristic notochord, nearly all the Chordata are marked by the presence, either in embryonic or larval stages only, or else persisting throughout life, of a number of slits or clefts in the walls of the pharynx which serve for breathing, and which are called gill-slits.