Type of Eumatozoa animals comprisiong three subphyla: tunicates (subphylum Tunicata), amphioxi (subphylum Cephalochordata) and vertebrates (subphylum Vertebrata). The vertebrates include: cyclostomes (order Cyclostomata, class Agnatha), Fishes, amphibians (class Amphibia), reptiles (class Reptilia), birds (class Aves), and mammals (class Mammalia). This is evolutionary the youngest group of animals. It is “only” 500 million years old, wheras the oldest invertebrate fossils are 1600 million years old and the earliest signs of life on Earth dates back some 3.5 billion years.

Chordates develop internal skeletons. The asiall part of the skeleton is a notochord present in the young of all chordates but possibly eliminated in adults. Above the notochord, that is on the back side of a body, a nerve cord is located that forms a nucleus of the nervous system. Digestive and circulatory systems are located on the other side of the notochord. Location of these four systems is characteristic to all chordates. Another characteristic of chordates is the existence of a common part of respiratory and digestive systems.

Chordates are a greatly varied type. They all reproduce sexually. The sexes are separated in most chordates but some of them are hermaphroditic. Very few chordates lead a sedentary life. An example of such an animal is a sea-squirt (class Ascidiacea) belonging to subphylum Tunicata.

Fishes are the most popular members of marine chordates. The vast majority of fishes have bony skeletons (class Osteichthyes) made of a bone tissue similar to the one found in humans, or cartilaginous skeletons (class Chondrichthyes) made of a cartilage tissue (cartilage tissue is found in humans, for example, at the thip of the nose). Sharks and rays are examples of fishes with cartilaginous skeletons.