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.
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 that cover its inside surface.
They have a diverticulum of the foregut called a stomochord, previously thought to be related with the chordate notochord, but this is most likely the result of convergent evolution rather than homology. 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 hemichordates give us the closest extant phylogenetic relative between the chordates and other invertebrates. Thus these marine worms, described to be the sister group of such animals as sea urchins, are of great importance to the scientific community interested in knowing the origins of chordate development. There are several species classified as hemichordates and there exists a moderate diversity of embryological development between these species. Hemichordates are classically known to develop in two ways both directly and indirectly. Hemichordates are a phylum composed of two classes the enteropneusts and the pterobranchs, both are forms of marine worm. The enteropneusts have two developmental strategies direct and indirect development. The indirect way of development is known to end in an extended pelagic plankotrophic tornaria larval stage, which means that this hemichordate exists in a larval stage that feeds on plankton before turning into an adult worm. Those species that are direct developing bypass this prolonged larval stage and develop directly into an adult worm. The following details the development of two popularly studied species of the hemichordata phylum Saccoglossus kowalevskii and Ptychodera flava. Saccoglossus kowalevskii is a direct developer and Ptychodera flava is an indirect developer. Most of what has been detailed in Hemichordate development has come from hemichordates that develop directly.
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