The phylum Chordata contains all animals that possess, at some point during their lives, a hollow nerve cord and a notochord, a flexible rod between the nerve cord and the digestive track. The phylum Chordata is an extremely diverse phylum, and the one most recognizable to us. The phylum contains about 43,700 species, most of them concentrated in the subphylum Vertebrata, making it the third-largest phylum in the animal kingdom. The phylum Chordata is divided into three subphylums: Urochordata (tunicates), Cephalachordata (lancelets), and Vertebrata (vertebrates). The first two phyla are very small containing only about 2,000 species total. Tunicates are marine animals that only show the attributes of the chordata phylum in the larva stage, and when they turn into adults lose the notochord and nerve cord. Adult tunicates look like small sacs around 3 cm tall attached to the ocean floor. Lancelets, which are similar in appearance to small fish, keep the nerve chord and notochord into maturity but are extremely simple in structure and lack a backbone.
The third phylum, vertebrata, is the most important, and is distinguished by a backbone (made either of bone or cartilage) containing interlocking vertebrae and a skull enclosing a brain. These two features serve to protect the entire central nervous system, and in addition give support and structure to the body; these bones also form part of a larger system of bones, the endoskeletal system. Unlike the exoskeleton of other phylums such as the arthropods, which must be shed periodically, this endoskeleton is permanent and can grow with the organism. This endoskeleton gives vertebrates a competitive edge over all other animals, as it can easily be scaled for use in large organisms, and it allows these organisms to be relatively light and fast-moving. In comparison, most organisms with an exoskeleton are small and slow-moving, due to the limitations of their large and bulky skeletal system.
Jawless fish are the most primitive vertebrates, and are similar to other fish with the exception that they have not evolved any sort of jaw, instead using a circular, suckerlike mouth to latch onto and suck the blood of their prey. An example of a jawless fish, the sea lamprey, is shown below (this picture also demonstrates the jawless fish's method of feeding). Eels are another example of jawless fish.
Cartilaginous fish have jaws, but have a skeleton made of flexible cartilage rather than bone (hence the name of the class), with only the teeth and sometimes the vertebrae containing calcium. Examples of cartilaginous fish are sharks, skates, and rays. Cartilaginous fish are often predators, and can be found in coeans throughout the world. Several examples of cartilaginous fish are shown below (from left to right, Blue shark, sawfish):
Bony fish are the largest class of vertebrate, with over 29,000 species, and have succeeded in a large variety of environments, including freshwater lakes, coral reefs, and deep-sea beds. Unlike cartilaginous fish, bony fish have a skeleton made of bone, and also have good eyesight, unlike that of cartilaginous fish, which is often very poor. In addition, bony fish have a special air-filled sac, the swim blatter, which allows them to remain buoyant. Bony fish can breathe without swimming because of a special flap of skin called the operculum, which covers the gills and when moved forces water over the gills. Examples of bony fish are salmon, cod, and sturgeon. An Atlantic salmon is shown below:
Amphibians consist of all four-legged vertebrate that do not lay amniotic eggs (meaning that the eggs do not contain a fluid-filled sac called the amnion surrounding the developing embryo). As a result, the eggs dry out quickly in the air, forcing all amphibians to lay their eggs in the water. The amphibian class is the smallest of the vertebrate classes, and includes about 4,000 species. Amphibians utilize lungs rather than gills for obtaining oxygen, and generally have soft skin. Amphibians usually spend some of their time on land and in water, though of course they must return to the water when they reproduce. Examples of amphibians are salamanders, newts, toads, and frogs. Several examples of amphibians are shown below (from left to right, Ensatina salamander, American toad):
Reptiles are the second of the four-legged vertebrate classes, and are much better adapted to living on land than amphibians. Some of these adaptations include amniotic eggs, tough skin coated by keratin, and a respiratory system with branching bronchial tubes in the lungs. Other characteristic features are the process of molting, whereby a reptile sheds its outer skin, teeth adapted for holding rather than chewing prey (reptiles swallow their prey whole rather than chew it), good hearing, and a tongue that can smell as well as taste. Snakes are also ectothermic, or cold-blooded, and must rely on natural conditions to maintain an optimum body temperature. Reptiles today encompass about 7,000 species, although before the extinction of the dinosaurs reptiles were the dominant vertebrate animal. Examples of reptiles are turtles, tortoises, crocodiles, snakes, and lizards. Several reptiles are shown below (from left to right, timber rattlesnake, nile crocodile, mud turtle):
Birds are mammals characterized by the presence of adaptations allowing flight (although not all birds have the ability to fly), and include about 10,000 species. These characteristics include feathers, forelimbs that have evolved into wings, hollow bones, and very few vertebrae to reduce weight. Birds also don't have teeth. In addition, birds have evolved an extremely specialized respiratory system including lungs with openings at both ends and air sacs to help in the movement of air through the lungs. These specializations allow birds to maintain the high levels of oxygen necessary to fuel their extremely fast metabolism. Birds are endothermic, and so heat themselves rather than relying on heat from their environment. Several examples of birds are shown below (from left to right, broad-tailed hummingbird, bald eagle, American robin):
Mammals are both the most diverse and the most advanced of all the groups in the kingdom Animalia, though they make up only about 4,600 species. Mammals are the only animals that nourish their young using milk produced by mammary glands, and also are the only animals that have teeth specilaized between species for different functions. In addition, mammals have skin covered by hair, which serves to insulate the body and regulate body temperature, a four-chambered heart and efficient circulatory system, four limbs, and highly developed brains. Mammals are warm-blooded (endothermic) and have a high metabolism similar to birds, allowing them to be active and move quickly. Mammals have adapted to live in a variety of environments, including on land throughout the world, in the air, and in the sea. Mammals make up the largest animals both on land and in the sea. This class includes animals as diverse as the platypus, koala, wolf, cat, cow, pig, whale, dolphin, bat, and human. Several examples of mammals are shown below (from left to right, duck-billed platypus, African elephant, Sumatran tiger, killer whale, gorilla, red fox):