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(Bateson, 1885 )

  • General Description
  • General Description

    Hemichordata is a small marine phylum of around a hundred species of worm-like, benthic deposit, or suspension feeders. They share some chordate characters, having a primitive form of notochord, (at least in early life), possibly a trait they share with the common ancestor of chordata, so are important in the study of vertebrate evolution. However, they are not classified as true chordates and DNA evidence suggests they are more closely related to echinoderms, supported by the similarity between some larval hemichordates and echinoderms.
    There are two classes, Pterobranchia and Enteropneusta which are very different, but are linked by some characters. The Pterobranchia, with around 20-30 species, are small tentaculate organisms, forming colonies with branching tubes and live in much deeper water than the Enteropneusts. They are difficult to study and little is known about their ecology. Larvae which are released have a very short pelagic existence, are ovoid, elongated, ciliated and ~ 400 µm in length. These unremarkable larvae are very unlikely to be caught in plankton samples, so will not be covered here.
    Figure 1. Hemichordata.
    The most familiar hemichordates are the Enteropneusta, commonly called acorn worms, of which there are only about 90 known species worldwide. The smallest are only a few millimetres long and the largest, can reach lengths of ~1.5 m. They are mainly found in shallow coastal waters, but occur down to a depth of 3,000 m. They live in U-shaped burrows on the sea-bed and their casts, left exposed at low tide, are a familiar sight in the tropics. The larvae of some species are found in the plankton.
    Further information: Burdon-Jones 1957; Hadfield 2002; Larink & Westheide 2006; Todd et al. 1991).
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    [HEMICHORDATA on the taxonomic tree]

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