Chordata  

Highly effective swimming: fish
 

Fish have effective maneuverability, braking, stability and thrust thanks to multiple fins.

 
  "By flexing the pectoral and caudal fins the fish can turn up, down, or sideways. The pectoral fins are also used as brakes, being pushed forwards like the flaps on an aircraft wing. The positions of the paired fins, especially the pelvic fins, are constantly adjusted to keep the fish from pitching or rolling: the pectorals tend to produce lift, which is counteracted by the downward thrust of the pelvics." (Foy and Oxford Scientific Films 1982:186)
  Learn more about this functional adaptation.

Barcode of Life Data Systems (BOLD) Stats
Specimen Records:432017
Specimens with Sequences:391292
Specimens with Barcodes:331659
Species:32563
Species With Barcodes:29752
Public Records:241174
Public Species:19118
Public BINs:26818

The Phylum Chordata includes the well-known vertebrates (fishes, amphibians, reptiles, birds, mammals). The vertebrates and hagfishes together comprise the taxon Craniata. The remaining chordates are the tunicates (Urochordata), lancelets (Cephalochordata), and, possibly, some odd extinct groups. With few exceptions, chordates are active animals with bilaterally symmetric bodies that are longitudinally differentiated into head, trunk and tail. The most distinctive morphological features of chordates are the notochord, nerve cord, and visceral clefts and arches.

Chordates are well represented in marine, freshwater and terrestrial habitats from the Equator to the high northern and southern latitudes. The oldest fossil chordates are of Cambrian age. The earliest is Yunnanozoon lividum from the Early Cambrian, 525 Ma (= million years ago), of China. This was just recently described and placed with the cephalochordates (Chen et al., 1995). Another possible cephalochordate is Pikaia (Nelson, 1994) from the Middle Cambrian. These fossils are highly significant because they imply the contemporary existence of the tunicates and craniates in the Early Cambrian during the so-called Cambrian Explosion of animal life. Two other extinct Cambrian taxa, the calcichordates and conodonts, are uncertainly related to other Chordata (Nelson, 1994). In the Tree of Life project, conodonts are placed as a subgroup of vertebrates.

Chordates other than craniates include entirely aquatic forms. The strictly marine Urochordata or Tunicata are commonly known as tunicates, sea squirts, and salps. There are roughly 1,600 species of urochordates; most are small solitary animals but some are colonial, organisms. Nearly all are sessile as adults but they have free-swimming, active larval forms. Urochordates are unknown as fossils. Cephalochordata are also known as amphioxus and lancelets. The group contains only about 20 species of sand-burrowing marine creatures. The Cambrian fossils Yunnanozoon and Pikaia are likely related to modern cephalochordates.

During the Ordovician Period (510 - 439 Ma) jawless or agnathan fishes appeared and diversified. These are the earliest known members of Vertebrata, the chordate subgroup that is most familiar to us. Fossils representing most major lineages of fish-like vertebrates and the earliest tetrapods (Amphibia) were in existence before the end of the Devonian Period (363 Ma). Reptile-like tetrapods originated during the Carboniferous (363 - 290 Ma), mammals differentiated before the end of the Triassic (208 Ma) and birds before the end of the Jurassic (146 Ma).

The smallest chordates (e.g. some of the tunicates and gobioid fishes) are mature at a length of about 1 cm, whereas the largest animals that have ever existed are chordates: some sauropod dinosaurs reached more than 20 m and living blue whales grow to about 30 m.

The notochord is an elongate, rod-like, skeletal structure dorsal to the gut tube and ventral to the nerve cord. The notochord should not be confused with the backbone or vertebral column of most adult vertebrates. The notochord appears early in embryogeny and plays an important role in promoting or organizing the embryonic development of nearby structures. In most adult chordates the notochord disappears or becomes highly modified. In some non-vertebrate chordates and fishes the notochord persists as a laterally flexible but incompressible skeletal rod that prevents telescopic collapse of the body during swimming.

The nerve cord of chordates develops dorsally in the body as a hollow tube above the notochord. In most species it differentiates in embryogeny into the brain anteriorly and spinal cord that runs through the trunk and tail. Together the brain and spinal cord are the central nervous system to which peripheral sensory and motor nerves connect.

The visceral (also called pharyngeal or gill) clefts and arches are located in the pharyngeal part of the digestive tract behind the oral cavity and anterior to the esophagus. The visceral clefts appear as several pairs of pouches that push outward from the lateral walls of the pharynx eventually to reach the surface to form the clefts. Thus the clefts are continuous, slit-like passages connecting the pharynx to the exterior. The soft and skeletal tissues between adjacent clefts are the visceral arches. The embryonic fate of the clefts and slits varies greatly depending on the taxonomic subgroup. In many of the non-vertebrate chordates, such as tunicates and cephalochordates, the clefts and arches are elaborated as straining devices concerned with capture of small food particles from water. In typical fish-like vertebrates and juvenile amphibians the walls of the pharyngeal clefts develop into gills that are organs of gas exchange between the water and blood. In adult amphibians and the amniote tetrapods (= reptiles, birds and mammals) the anteriormost cleft transforms into the auditory (Eustachian) tube and middle ear chamber, whereas the other clefts disappear after making some important contributions to glands and lymphatic tissues in the throat region. The skeleton and muscles of the visceral arches are the source of a great diversity of adult structures in the vertebrates. For example, in humans (and other mammals) visceral arch derivatives include the jaw and facial muscles, the embryonic cartilaginous skeleton of the lower jaw, the alisphenoid bone in the side wall of the braincase, the three middle ear ossicles (malleus, incus and stapes), the skeleton and some musculature of the tongue, the skeleton and muscles of the larynx, and the cartilaginous tracheal rings.