The Phylum Chordata, the chordates, is the part of the Animal Kingdom containing organisms that possess a structure called a notochord, at least during some part of their development. The best known chordates are the vertebrates (fishes, amphibians, reptiles, birds, mammals). The vertebrates and hagfishes together comprise the taxon Craniata. Also included in the phylum are the tunicates (Urochordata) and lancelets (Cephalochordata). Some extinct groups are also assigned to this phylum.
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Cephalochordates are non-vertebrate members of the Phylum Chordata, having diverged from the evolutionary line leading to the vertebrates before the end of the Precambrian.
A cephalochordate has no head, a transparent body like a leaf with a notochord running from end to end, and rows of muscle on either side by which to wriggle, swim or bury itself. They are commonly known as lancelets.
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Aquatic chordates with appendages developed as fins, whose primary respiratory organs are gills and whose body is usually covered with scales.
Websites about keeping or caring for turtles in captivity, including pet turtles, should be submitted to http://dmoz.org/Recreation/Pets/Herp/Reptiles/Turtles_and_Tortoises .
Tributes to wild turtles, websites about watching turtles in the wild, and other websites devoted to a recreational appreciation of turtles should be submitted to http://dmoz.org/Recreation/Outdoors/Wildlife/Turtles/ .
The subphylum Urochordata, is also sometimes known as the Tunicata. These animals are commonly known as tunicates or sea squirts. Some pelagic tunicates are known as salps. There are two classes recognized within the subphylum: Thaliacea and Ascidiacea.
The body of an adult tunicate is quite simple. Essentially it is a sack with two siphons through which water enters and exits. Many tunicates have a larva that is free-swimming and exhibits all chordate characteristics, including a notochord. In many tunicates this "tadpole larva" eventually attaches to a hard surface and loses its tail, the ability to move, and most of its nervous system. The tunicate group knows as salps are entirely free-swimming, however.
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