Hemichordata

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Related to hemichordates: Cephalochordates, Urochordates

Hemichordata

(hĕmĭkôr'dā`tə), small phylum of marine invertebrates closely related to both the echinoderms (phylum EchinodermataEchinodermata
[Gr.,=spiny skin], phylum of exclusively marine bottom-dwelling invertebrates having external skeletons of calcareous plates just beneath the skin. The plates may be solidly fused together, as in sea urchins, loosely articulated to facilitate movement, as in sea
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) and chordates (phylum ChordataChordata
, phylum of animals having a notochord, or dorsal stiffening rod, as the chief internal skeletal support at some stage of their development. Most chordates are vertebrates (animals with backbones), but the phylum also includes some small marine invertebrate animals.
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). Acorn worms, class Enteropneusta, are the most common hemichordates. The body is composed of an anterior, conical proboscis, a short collar, and a long, wormlike trunk. Gill clefts in the pharynx and a hollow nerve cord in the collar suggest relationship to chordates, while the ciliated larvae are similar to those of echinoderms. Hemichordates live in burrows or under objects in shallow water and feed on detritus. Balanoglossus gigas may reach a length of nearly 5 ft (1.5 m), but most species are considerably smaller.

Also in the phylum is the class Pterobranchia, whose members are tiny deep-sea creatures that form colonies by secreting a collagenous exoskeleton. They are considered more primitive than the enteropneusts. Pterobranchs have been identified with graptolites, hemichordates of the class Graptolithina (formerly believed to be extinct) since the discovery in 1992 of a living pterobranch identical to fossil graptolites.

Hemichordata

 

a phylum of bottom-dwelling marine invertebrates of the group Deuterostomata. The Hemichordata were previously considered a subphylum of Chordata because of the presence of a chordlike organ (notochord), the dorsal location of the nerve stem, the development of gill slits, and several similar features of embryonic development. However, unlike the chordates, hemichordates have a three-segmented body consisting of a proboscis, collar cavities, and trunk cavities. Modern-day hemichordates are the Enteropneusta and Pterobranchia; fossil representatives include the graptolites.

Hemichordata

[¦he·mē·kȯr′däd·ə]
(systematics)
A group of marine animals categorized as either a phylum of deuterostomes or a subphylum of chordates; includes the Enteropneusta, Pterobranchia, and Graptolithina.
References in periodicals archive ?
Hemichordates also share many of the same characteristics as chordates - a group of animals that includes humans - with the name hemichordate roughly translating to 'half a chordate.'
Head regeneration in hemichordates is not a strict recapitulation of development.
2), but in contrast with the Cambrian, in the modern community polychaetes are the second group in importance instead of hemichordates. B.
Biomineral ultrastructure, elemental constitution and genomic analysis of biomineralization-related proteins in hemichordates. Proc.
We are indebted to .Saskia Brix, the IceAGE Project research team, and Senckenberg's German Center for Marine Biodiversity Research (DZMB) for support in collecting Icelandic hemichordates. Many thanks to the staff and crews of R/V Meteor, R/V N.
A 95% majority rule consensus tree for hemichordates (40 million generations) was constructed from concatenated, but unlinked nucleotide sequences (with no regions excluded) in MrBayes 3.2.1 run on the Smithsonian Institution's Hydra cluster in replicate.
Deuterostomia is a super phylum that encompasses hemichordates (Enteropneusta), echinoderms (sea urchins), and chordates (vertebrates); along with the phylum protostomia, it represents all animals having bilateral symmetry at least early in development (Fig.
It has been considered to correspond, at least in part, to the protocoel of hemichordates (MacBride, 1909), the original foregut of chordates (van Wijhe, 1913), the pre-premandibular head cavities of craniates (Neal, 1915), or the premandibular head cavities plus part of the adeno-hypophysis of craniates (Goodrich, 1917).
Convergence in the feeding apparatuses of lophophorates and pterobranch hemichordates revealed by 18S rDNA: an interpretation.
This is accomplished by a single-band upstream particle-capture system in echino-derms and hemichordates, but by an opposed-band feeding system in several lophotrochozoan phyla (Strathmann, 1971; Strathmann et al., 1972; Hart, 1991).
A distinctive nerve cell type common to diverse deuterostome larvae: comparative data from echi- noderms, hemichordates and amphioxus.