AskDefine | Define hemichordate

Extensive Definition

Hemichordata is a phylum of worm-shaped marine deuterostome animals, generally considered the sister group of the echinoderms. 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, formed from a diverticulum of the foregut called a stomochord, 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.
The Body of Hemichordates are divided into three parts, proboscis, collar and trunk. They have open circulatory systems also complete digestive tract but the musculature in their gut is very poorly developed, and food is mostly transported through it by using the cilia that cover its inside surface.
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. One of the suggestions is that the pterobranchs are more basal deuterostomes, while the enteropneusts are an early offshoot of the lineage who are leading to Chordata.

References

hemichordate in Arabic: نصف حبليات
hemichordate in Catalan: Hemicordat
hemichordate in Czech: Polostrunatci
hemichordate in German: Kiemenlochtiere
hemichordate in Estonian: Ürgkeelikloomad
hemichordate in Spanish: Hemichordata
hemichordate in French: Hemichordata
hemichordate in Korean: 반색동물
hemichordate in Croatian: Žiroglavci
hemichordate in Italian: Hemichordata
hemichordate in Lithuanian: Nepilnachordžiai
hemichordate in Hungarian: Félgerinchúrosok
hemichordate in Macedonian: Полухордати
hemichordate in Dutch: Hemichordata
hemichordate in Japanese: 半索動物
hemichordate in Occitan (post 1500): Hemichordata
hemichordate in Polish: Półstrunowce
hemichordate in Portuguese: Hemichordata
hemichordate in Russian: Полухордовые
hemichordate in Slovak: Polochordáty
hemichordate in Serbian: Полухордати
hemichordate in Finnish: Esiselkäjänteiset
hemichordate in Swedish: Svalgsträngsdjur
hemichordate in Vietnamese: Ngành Động vật nửa dây sống
hemichordate in Turkish: Yarı sırtipliler
hemichordate in Zeeuws: Hemichordata
hemichordate in Chinese: 半索动物门
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