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 ?
One of the big punchlines from my graduate work, was molecular evidence that enteropneusts and pterobranchs are closely related," said Cameron, a specialist on the taxonomy, evolution and biogeography of hemichordates.
Although the two phyla appear quite different today, chordates are often linked with the hemichordates and another dissimilar phylum, the echinoderms, which include starfish.
The creature belongs to a group called pterobranch hemichordates which are related to starfish and sea urchins but also show some characteristics that offer clues to the evolution of the earliest vertebrates.
In preliminary studies, Shu Degan of Xian Northwest University places Yunnanozoon in another phylum, the hemichordates, which includes the modern acorn worms.