Chordates are a diverse group of animals that comprise the phylum Chordata. The phylum is divided into three subphyla based on general physical characteristics: Urochordata, Cephalochordata, and Vertebrata. Urochordates have a notochord (a hollow dorsal nerve cord) while in the larvae stage but lose it later in their adult stage. Cephalochordates have both a notochord and a nerve cord but do not have vertebrae. Vertebrates, as the name implies, have vertebrae.
There are approximately 44, 000 species of chordates, ranging in size from several millimeters to 105 ft (32 m) long. The simplest and earliest chordates are pre-vertebrate animals such as ascidians, tunicates, and Amphioxus. The major group of chordates is the sub-phylum Vertebrata, the vertebrates. Listed more-or-less in the order of their first appearance in the fossil record, vertebrates include sharks, lampreys, bony fishes, amphibians, reptiles, birds, and mammals.
Chordates exhibit bilateral symmetry, and they have a body cavity (the coelom), which is enclosed within a membrane (the peritoneum), and which develops from the middle tissue layer known as the mesoderm. A defining feature of chordates is a structure known as the notochord. This is a rod-like flexible structure that runs along the upper, mid-line of chordates. A notochord is present for at least some part of the life of all chordates. In the earliest chordates, the notochord stiffens the body against the pull of muscles. This function is less important in the more advanced vertebrate chordates, whose bodies are supported by the cartilaginous and bony elements of the skeleton. In vertebrates, the notochord is only present during the embryonic, developmental stages.
Other defining features of chordates are the presence of pharyngeal gill slits (which are precursors of the gill arches in fish and amphibians), hollow nerve cords on the upper surface of the animal (that eventually develop into the spinal cord in vertebrates) and a tail extending beyond the anal opening. As with the notochord, these features may only occur during a part of the life of the animal, especially in the more recently evolved chordates, such as the vertebrates.
Chordate animals have a closed circulatory system, in which blood is transported around the body inside veins and arteries of various sizes. The blood is circulated by the pumping action of the heart; the respiratory gases in the blood diffuse across the thin walls of the smallest vessels (capillaries) in the tissues. The most recently evolved vertebrates have a four-chambered heart and a double circulation of the blood, which involves a separate circulation for the heart and the lungs and for the heart and the rest of the body (systemic circulation).
Most chordates have two sexes, and the male and female individuals tend to be different in many aspects of their form and function (dimorphic). Fertilization is external in the earlier-evolved groups of chordates (fish and amphibians) and internal in later groups (reptiles, birds, and mammals). Many chordates lay eggs, or are oviparous, while others give birth to live young (viviparous).
Chordates utilize a wide range of habitats. The earliest evolved chordates and some of the more recent groups are aquatic, while others are primarily terrestrial.
See also Sea squirts and salps.