Hemichordata

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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.'
Ptychoderid hemichordate neurulation without a notochord.
cDNA sequences for transcription factors and signaling proteins of the hemichordate Saccoglossus kowalevskii: efficacy of the expressed sequence tag (EST) approach for evolutionary and developmental studies of a new organism.
Complete morphological species descriptions are forthcoming, but herein we focused on molecular data in order to provide a more comprehensive overview of unknown hemichordate diversity.
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.
Although high-speed recordings are lacking for ciliary feeding by larvae of other echinoderm classes and of hemichordates, the morphology of ciliary bands and the particle paths near feeding and non-feeding larvae suggest that these larvae also capture particles by an induced local reversal of beat (Strathmann, 1971; Strathmann and Bonar, 1976; Hart, 1991, 1996b; Hart et al., 1994).
1B) and fecal mounds of hemichordates. Snails like the scavenger Nassarius luteostoma and the predator Natica unifasciata crawl on the sediment surface year around, while the sand dollar Encope stokessi is more frequent during the dry seasons.
The chordates (that include vertebrates), hemichordates (that include the living "acorn worms"), and echinoderms (that include the living starfish and echinoids) are all deuterostomes and have the same pattern of early embryo development.
There is much interest in these deep sea hemichordates since they are close to the evolutionary link between vertebrates and invertebrates.
This animal is thought to share affinities with both echinoderms (a group that includes the starfish) and a small group of marine animals called hemichordates, but the exact connection remains difficult to establish.
Nonfeeding larvae have evolved from feeding larvae many times within the echinoderms, hemichordates, lophophorates, polychaetes, gastropods, bivalves, crustaceans, and other taxa (Strathmann 1978).