of sperm whales. Various aspects of thebiology of deep-water squids are dis-cussed in “Adaptations of Midwater Animals” (p.361).Cuttlefishes (Sepia ) resemble squidsin having eight arms and two tentacles,but the body is flattened and has a finrunning along the sides. Cuttlefishes, which are not fish at all, have a calcifiedinternal shell that aids in buoyancy. Thisshell is the “cuttlebone” sold as a sourceof calcium for cage birds. An unusual external shell character-izes the chambered nautilus ( Nautilus;see“The Chambered Nautilus,” p.376). Theimpressive shell is smooth, coiled, and upto 25 cm (10 in) in diameter. The shellcontains a series of gas-filled chambersthat serves as a buoyancy organ. Thebody—which occupies the outer, largestchamber—has 60 to 90 short, suckerlesstentacles used to capture crabs and fish.