Subphylum Vertebrata. This group includes animals with vertebrae. All but the classes Aves and Mammalia are cold-blooded.
Class Cyclostomata. The hagfishes and lampreys are fishlike forms but without paired fins. They have a circular sucking mouth without jaws. The former are all marine, while the latter live both in the sea and in fresh water.
Class Elasmobranchii. These primitive fishes—the sharks, rays, and chimaeras with a cartilaginous endoskeleton—have paired fins and a lower jaw. In this group are many large forms such as the giant manta and the whale shark, the largest of all fishes, which becomes about 16 m long. Nearly all are marine.
Class Pisces. This class includes the true fishes, with a bony endoskeleton, paired fins, and an operculum covering the gills. They are characteristically streamlined for great swimming
Class Reptilia. This class is represented in the sea by snakes and turtles. They breathe air and are therefore inhabitants of surface waters. The turtles frequent the shore to deposit their eggs on sandy beaches; the snakes bring forth living young and are therefore less dependent upon the shore. The sea snakes are found in the Indo-West Pacific and in tropical waters of America. They grow to a length of from 1 to 2 m or more and some are very poisonous. The sea turtles occur in tropical and subtropical seas. They have paddlelike limbs for swimming, and some grow to great size. The leathery turtle, for example, which is the largest of the class, may attain a weight of 1000 pounds.
Class Aves. A great number of birds are dependent upon the sea for food. Some of these frequent the land only for nesting and rearing of young. Typical examples are the albatrosses, petrels, cormorants, and auks.
Class Mammalia. These are warm-blooded, air-breathing animals with hair and mammary glands.
Order Carnivora. The marine members of this order are the sea otters and, to a lesser degree, the polar bears. The sea otters occur only in small numbers and only along the west coast of North America, where they were formerly hunted commercially to the very verge of extinction. Recently, under rigid protection, they have recuperated to an encouraging degree. The polar bears are confined to the Arctic region, usually on or near floating ice
Order Pinnipedia. Pinnipedia include seals and walruses, nearly all marine. The limbs are finlike, in adaptation to the aquatic existence. There are three families: (1) Otariidae include the eared seals, sea lions, and fur seals. Small external ears are present and the hind limbs can be rotated forward. (2) Phocidae are the hair seals without external ears and with hind limbs incapable of rotation forward. (3) Odobenidae include the walruses, with greatly elongated canine teeth in the upper jaw. They are confined to the Arctic.
Order Sirenia. Sirenia are heavy-bodied mammals with a flat tail and with forelimbs modified as paddles. Hind limbs are wanting. They live near shores in warm waters,― 314 ―where they browse upon vegetation. They are not numerous. Examples: sea cows, manatees, and dugongs.
Order Cetacea. This order includes whales and dolphins, highly modified for aquatic life by a streamlined body and finlike forelimbs and tail. The hind limbs are wanting.
Suborder Mysticeti. These are the baleen, or whalebone, whales, with a series of long plates of baleen suspended in the mouth (fig. 76a). The frayed ends of these are used in screening out plankton food. Examples: fin whale, humpbacked whale, and blue whale. The last named is the largest of all animals, growing to a maximum length of about 34 m and weighing 294,000 pounds.
a, the blue whale—a whalebone whale; b, the sperm whale—a toothed whale.
Suborder Odontoceti. Odontoceti are the toothed whales. This group includes (1) sperm whales with teeth only in the lower jaw (fig. 76b) and (2) the numerous dolphins and porpoises with teeth in both jaws.