Vertebrates, animals with backbones. The vertebrates include humans and other mammals, birds, reptiles, amphibians, and fishes. The first vertebrates, the jawless fishes, appeared on the earth about 500,000,000 years ago.
The backbone, or spinal column, is made up of a number of vertebrae (singular, vertebra) of bone or cartilage joined to form a flexible column. .) Its forward end is connected to a bony or cartilaginous (gristly) skull that protects the brain. Other bones are connected to the backbone and to one another to form an inner skeleton, or endoskeleton, that is covered with skin. (Animals such as insects and spiders have external skeletons, or exoskeletons.)
A vertebrate has a central nervous system made up of a brain and a spinal cord. The spinal cord extends from the brain through the spinal column. A vertebrate also has bilateral symmetry (the right and left sides of the body are closely alike). Most vertebrates have two pairs of limbs. In humans these limbs are the arms and legs. In birds they are the wings and legs. In fishes they are the pectoral and pelvic fins. Some vertebrates have only one pair of limbs, and limbs are entirely absent in most snakes, and some other vertebrates. (A few snakes, including anacondas, boa constrictors, and pythons, have vestigial hind limbs in the form of short spurs.)
Vertebrates form part of a phylum of animals called chordates. All chordates possess a notochord and gills at some stage of their development. The notochord is a flexible supporting rod. Primitive chordates retain it throughout their lives. In vertebrates it is replaced or supplemented by the backbone. Fishes breathe through gills all their lives. Gills of amphibians (frogs, toads, salamanders) evolved separately from those of fishes. Most amphibians lose their gills when they mature. In the higher vertebrates gills are present in embryos, but are closed at birth.
Vertebrates form the subphylum Vertebrata, or Craniata, of the phylum Chordata. The subphylum Vertebrata is divided into seven classes:
1. Cyclostomata, or Agnatha, lampreys and hagfishes. Cyclostomates have a body that somewhat resembles that of an eel. They have neither jaws nor paired fins. Their skeletons are gristly, and they have notochords, supplemented by spinal columns, all their lives. A lamprey has seven uncovered gill slits. A hagfish has six gill slits covered by a fold of skin.
2. Elasmobranchii, or Chondrichthyes, sharks, skates and rays, and chimaeras. These marine animals have jaws, gristly skeletons, and lifelong notochords that are partly replaced by vertebrae. The gill slits are exposed in sharks and in skates and rays; they are covered by folds of skin in the chimaeras. All elasmobranchids have paired fins, two-chambered hearts, and toothlike scales. Each has the mouth on the underside of the body and a spiral valve in the rectum. None has a swim bladder.
3. Teleostomi, or Osteichthyes, the bony fishes. The skeletons of teleosts are usually bony or partly bony, but they are wholly gristly in sturgeons and paddlefish. Most teleosts have scales, though not of the toothlike type, and gills covered by horny plates. They have two-chambered hearts and normally two pairs of paired fins. Many have swim bladders and primitive species have lungs.
4. Amphibia, frogs, toads, salamanders, and caecilians. Amphibians are cold-blooded animals, usually four-legged, with naked, slimy skins. Nearly all hatch from jelly-covered eggs laid in water. The young have gills, and some adults retain the gills. Most adults, except lungless salamanders, have lungs, and most breathe partly or wholly through the skin. All have three-chambered hearts with two auricles and one ventricle.
5. Reptilia, lizards, snakes, alligators, crocodiles, and turtles. Reptiles are cold-blooded animals with scaly skins. Except for snakes and some lizards, they are four-legged, with five toes on each foot, usually with claws. Many spend much time in water, but all living species are true land animals. All have lungs, and all, even sea turtles, lay their eggs on land. Reptiles have three-chambered hearts; in some, the heart has a partly divided ventricle, almost forming a fourth chamber. The skull is attached to the backbone at a single point.
6. Aves, birds. These are warm-blooded animals with feathers, wings, beaks, and four-chambered hearts. Most of them are able to fly. They have light, often hollow, bones. Air sacs extend from the lungs into the hollow bones and the body cavity. Birds lay eggs with brittle shells.
7. Mammalia, mammals (animals that nurse their young). Mammals are warm-blooded and have at least some hair on their bodies. Their brains are well-developed, and their hearts have four chambers. A diaphragm separates the chest from the abdomen. Most mammals have four limbs with nails, claws, or hoofs on the digits. The skull is attached to the backbone at two points. Except for monotremes, the young of all mammals develop within the body of the mother. Monotremes lay eggs. The young of marsupials complete their development from an early stage attached to the nipple of the mother, usually within an abdominal pouch.