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Hemichordata is a phylum of worm-shaped marine deuterostome
Deuterostome

Deuterostomes are a superphylum of animals. They are a taxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes....
 animal
Animal

Animals are a major group of multicellular, eukaryotic organisms of the Kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life....
s, generally considered the sister group of the echinoderm
Echinoderm

Echinoderms are a Phylum of Marine animals . Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone.Aside from the problematic Arkarua, the first definitive members of the phylum appeared near the start of the Cambrian period....
s. They date back to the Lower or Middle Cambrian
Cambrian

The Cambrian is a geologic period that began about Mya at the end of the Proterozoic eon and ended about Ma with the beginning of the Ordovician period ....
 and include an important class of fossil
Fossil

Fossils are the preserved remains or trace fossil of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous Rock formations and sedimentary rock layers is known as the fossil record....
s called graptolite
Graptolite

Graptolites are fossil colonial animals known chiefly from the Upper Cambrian through the Lower Carboniferous . A possible early graptolite, Chaunograptus, is known from the Middle Cambrian....
s, most of which became extinct in the Carboniferous
Carboniferous

The Carboniferous is a geologic period that extends from the end of the Devonian period, about 359.2 ? 2.5 annum , to the beginning of the Permian period, about 299.0 ? 0.8 Ma ...
. They seem to have a primitive form of notochord
Notochord

The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cell s derived from the mesoderm and defines the primitive axis of the embryo....
, formed from a diverticulum of the foregut called a stomochord, but this is most likely the result of convergent evolution
Convergent evolution

Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action....
. 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 bodies of Hemichordates are divided into three parts, proboscis, collar and trunk.






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Hemichordata is a phylum of worm-shaped marine deuterostome
Deuterostome

Deuterostomes are a superphylum of animals. They are a taxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes....
 animal
Animal

Animals are a major group of multicellular, eukaryotic organisms of the Kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life....
s, generally considered the sister group of the echinoderm
Echinoderm

Echinoderms are a Phylum of Marine animals . Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone.Aside from the problematic Arkarua, the first definitive members of the phylum appeared near the start of the Cambrian period....
s. They date back to the Lower or Middle Cambrian
Cambrian

The Cambrian is a geologic period that began about Mya at the end of the Proterozoic eon and ended about Ma with the beginning of the Ordovician period ....
 and include an important class of fossil
Fossil

Fossils are the preserved remains or trace fossil of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous Rock formations and sedimentary rock layers is known as the fossil record....
s called graptolite
Graptolite

Graptolites are fossil colonial animals known chiefly from the Upper Cambrian through the Lower Carboniferous . A possible early graptolite, Chaunograptus, is known from the Middle Cambrian....
s, most of which became extinct in the Carboniferous
Carboniferous

The Carboniferous is a geologic period that extends from the end of the Devonian period, about 359.2 ? 2.5 annum , to the beginning of the Permian period, about 299.0 ? 0.8 Ma ...
. They seem to have a primitive form of notochord
Notochord

The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cell s derived from the mesoderm and defines the primitive axis of the embryo....
, formed from a diverticulum of the foregut called a stomochord, but this is most likely the result of convergent evolution
Convergent evolution

Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action....
. 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 bodies of Hemichordates are divided into three parts, proboscis, collar and trunk. They have open circulatory systems and a complete digestive tract but the musculature in their gut is very poorly developed, and food is mostly transported through it by using the cilia
Cilium

A cilium is an organelle found in eukaryote cell s. Cilia are tail-like projections extending approximately 5?10 micrometres from the cell body....
 that cover its inside surface.

Hemichordata are divided into two classes: the Enteropneusta, commonly called acorn worms, and the Pterobranchia
Pterobranchia

Pterobranchia is a clade of small, worm-shaped animal. They belong to the hemichordata, and live in secretion tubes on the ocean floor. Pterobranchia feed by filtering plankton out of the water with the help of cilia attached to tentacles....
, which may include the graptolite
Graptolite

Graptolites are fossil colonial animals known chiefly from the Upper Cambrian through the Lower Carboniferous . A possible early graptolite, Chaunograptus, is known from the Middle Cambrian....
s. 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.