Chordata is a phylum of animals that includes the vertebrates, Lancelets, and Tunicates. The Chordates are distinguished from other phyla by the presence of a notochord, which is present in all chordates during some stage of their life cycle. In most cases, the notochord is replaced by a vertebral column during embryonic development.
The phylum Chordata is divided into three subphyla: Vertebrata (which contains the fishes, amphibians, reptiles, birds, and mammals), Tunicata (which contains the tunicates), and Cephalochordata (which contains the lancelets).
The Vertebrata are further subdivided into five classes: Fish (including jawless fish, cartilaginous fish, and bony fish), Amphibia (frogs and toads), Reptilia (turtles, snakes, lizards, crocodiles), Aves (birds), and Mammalia (mammals). Each of these classes has distinct characteristics that will be discussed in more detail below.
Fish are aquatic chordates that lack limbs with digits. Jawless fish were the first fish to evolve and include species such as hagfish and lampreys. Jawless fish have a skeleton made of cartilage rather than bone and they do not have a true jaw. Instead, they have an oral disc surrounded by keratinized plates used for feeding. Cartilaginous fish are also aquatic but they do have jaws containing teeth. Examples of cartilaginous fish include sharks and rays. Bony fish are also aquatic chordates with jaws containing teeth. They have a skeleton made mostly of bone tissue as well as scales on their skin for protection. Bony fish include familiar species such as trout and salmon.
Amphibians are tetrapods—chordates with four limbs—that spend part of their life cycle in water and part on land. Frogs and toads are examples of amphibians. Most amphibians begin their lives as larvae living in water where they breathe through gills. As they mature, they undergo metamorphosis—a change in form—and develop lungs so that they can live on land where they typically breathe through their skin instead of gills.* During metamorphosis Amphibian larvae typically transform into an adult form that resembles the larval form except that it has lost its tail fin*and gained legs for walking on land.* Reptiles are another group of tetrapods.*They share many characteristics with amphibians but differ in several important ways.* For example*, reptiles laid shelled eggs*that protect developing embryos from drying out.* All reptiles are ectothermic**– meaning “outside heat” – relying on external sources of heat to regulate their body temperature*. This makes them well-suited to live in hot climates.* Snakes*, lizards*, turtles*, crocodiles*,and dinosaurs*are all examples of reptiles.* Birds evolved from a group of small feathered dinosaurs known as Theropods*. Like all other chordates*, birds have a backbone made up of vertebrae*. However*, birds also have feathers*and wings*. These adaptations allow them to fly – something no other chordate can do! Birds range in size from tiny hummingbirds to gigantic ostriches*. Mammals evolved from reptilian ancestors*during the Mesozoic Era*. One key difference between mammals and other chordates is that mammals nourish their young with milk produced by mammary glands*. Another difference is that hair covers the body surfaceof most mammals *. This provides insulation against cold weatherand helps animals like camels* store waterin hot climates *. Thereare two main groups o