Chordates include animals with backbones (vertebrates) and some related invertebrates. At some time in their life, chordates all possess a stiff rod, called a "notochord". This lies above the gut. In vertebrates, a series of bones (vertebrae) replace the notochord.
Two sub-phyla of invertebrate chordates exist:
  • Tunicata (sea squirts). These become highly modified as adults and attach to a substrate. They feed with gill slits. Only the free swimming larvae retain a notochord. They are aquatic species. Approximately 1,300 species are still in existence.
  • Cephalochordata (lancelets). These resemble very simple fish. They live in the sea and extract food from the water with their gill slits. Approximately 25 species exist.

The remaining Chordata belong to the sub-class vertebrata, and have a backbone. Of the approximately 42,000 species known, half of them are fish. Therefore, although the group is successful, it is outnumbered by arthropods and molluscs. Seven classes of this phylum are generally recognised:

  • Agnatha (Jawless fish such as lampreys). This is a superclass of around 100 species of jawless fish. These fish are also characterised by the absence of paired fins.
  • Chondrichthyes (Cartilaginous fish such as sharks and rays). These are jawed fish with paired fins, a two-chambered heart and most importantly a skeleton made of cartilage instead of bone.
  • Osteichthyes (Bony fish) This is the major class of fish and the largest vertebrate class containing over 29,000 species. It is divided into ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii)
  • Amphibia. (Amphibians including semi-aquatic animals including salamanders, newts, toads and frogs) It contains around 4,000 species characterised by being ectothermic (cold-blooded) animals that have two distinct life stages. One in water (eg. tadpole) and one on land.
  • Reptilia (Reptiles including turtles, snakes. lizards and crocodilian species) This class contains just under 8,000 ectothermic species that have horny scales and lay leathery eggs covered by an extra membrane.
  • Aves (Birds) This class is noteworthy not only for flight, but for warm blood and an insulating layer of feathers.
  • Mammalia (Mammals) This class is characterised by the possession of hair and mammary glands that secrete milk. Mammals are endothermic (warm blooded) and include humans.