Hemichordate: Difference between revisions
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'''Hemichordata''' is a [[Phylum (biology)|phylum]] of worm-shaped marine [[deuterostome]] [[animal]]s, generally considered the sister group of our own, the [[chordate]]s. They date back to the Lower or Middle [[Cambrian]] and include an important class of [[Fossil|fossils]] called [[graptolite]]s, most of which became extinct in the [[Carboniferous]]. They seem to have a primitive form of |
'''Hemichordata''' is a [[Phylum (biology)|phylum]] of worm-shaped marine [[deuterostome]] [[animal]]s, generally considered the sister group of our own, the [[chordate]]s. They date back to the Lower or Middle [[Cambrian]] and include an important class of [[Fossil|fossils]] called [[graptolite]]s, most of which became extinct in the [[Carboniferous]]. They seem to have a primitive form of [[notochord]], but this is most likely the result of [[convergent evolution]]. A hollow neural tube exists among some species (at least in early life), probably a primitive trait they share with the common ancestor of chordata and the rest of the deuterostomes. |
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Hemichordata are divided into two classes: the [[Enteropneusta]], commonly called acorn worms, and the [[Pterobranchia]], which may include the graptolites. A third class, [[Planctosphaeroidea]], is proposed based on a single species known only from larvae. The phylum contains about 100 living species. The exact taxonomic position of hemichordata and whether the group is [[monophyletic]] is currently under debate. |
Hemichordata are divided into two classes: the [[Enteropneusta]], commonly called acorn worms, and the [[Pterobranchia]], which may include the graptolites. A third class, [[Planctosphaeroidea]], is proposed based on a single species known only from larvae. The phylum contains about 100 living species. The exact taxonomic position of hemichordata and whether the group is [[monophyletic]] is currently under debate. |
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Revision as of 21:22, 19 March 2006
| Hemichordata | |
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| File:Acorn worm.PNG | |
| a species of the genus Saccoglossus | |
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| Phylum: | Hemichordata Bateson, 1885
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Hemichordata is a phylum of worm-shaped marine deuterostome animals, generally considered the sister group of our own, the chordates. They date back to the Lower or Middle Cambrian and include an important class of fossils called graptolites, most of which became extinct in the Carboniferous. They seem to have a primitive form of notochord, but this is most likely the result of convergent evolution. A hollow neural tube exists among some species (at least in early life), probably a primitive trait they share with the common ancestor of chordata and the rest of the deuterostomes.
Hemichordata are divided into two classes: the Enteropneusta, commonly called acorn worms, and the Pterobranchia, which may include the graptolites. A third class, Planctosphaeroidea, is proposed based on a single species known only from larvae. The phylum contains about 100 living species. The exact taxonomic position of hemichordata and whether the group is monophyletic is currently under debate.