Hemichordate
| Hemichordata | |
|---|---|
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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 primtive form of a chordata, but this is most likely the result of parallel evolution. A hollow neural tube among some species (at least in early life), a 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. The phylum contains about 100 living species. The exact taxonomic position of hemichordata and whether the group is monophyletic is currently under debate.