The Vertebrates (Phylum Chordata, subphylum vertebrata)
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The Vertebrates (Phylum Chordata, subphylum vertebrata) |
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One subphylum of the chordates, the vertebrata, includes some of the most familiar of all animals, - including mammals , birds , reptiles , amphibians , and fishes. Also included in the phylum chordata are two
invertebrate phyla, the Urochordata and Cephalochordata , which together with the vertebrates comprise approximately 45,000 living species of Chordates .
The vertebrates share all the characteristics of chordates (including the invertebrate forms), which are:
In addition, the vertebrates possess a vertebral column (a backbone or spinal column), which are skeletal elements (the vertebrae) that run dorsally along the anterior-posterior axis and enclose and protect the dorsal nerve cord (also called the spinal cord in vertebrates). Although the evolutionary history of the chordates is controversial, they appear to have a common ancestor with most other deuterostomes , such as the echinoderms , lophophorates , and hemichordates . The chordates are sometimes placed in the superphylum Deuterostomia , which also includes the phyla Chaetognatha , Hemichordata , and Echinodermata . Pharyngeal gill slits may have originated in some early deuterostomes as a means of pumping water through the mouth and out the gill slits in order to increase respiratory gas exchange. Primitive chordates may have then modified these gill slits to function as filter feeding devices. Along with other basic features of the phylum, early chordates were thought to have a tadpole-like larval form, similar to that possessed by many modern day tunicate larvae. The tunicates appear to have been early evolutionary offshoots of the chordate line that lead to the other vertebrate groups. There are other theories of chordate origins, including close relationships with other groups such as arthropods and mollusks , however, the echinoderm - chordate theory is presently the most accepted view. At least some forms of vertebrates can be found throughout all depths and types of marine habitats, (as well as most terrestrial and freshwater habitats) that can support life. Fishes, mammals , and birds can be found worldwide in all seas and oceans. Some forms, such as the reptiles , are somewhat more limited in their distribution, and there are no truly marine amphibians . The Phylum Chordata includes:
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