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 ?
1928 242,126 Hemichordata Balanoglossus aurantiaca Girard, 1853 143,815 Cephalodisctts gracilis Harmer, 1905 57,139 Cephalodiscus hodgsoni Ridewood, 1907 200,052 Cephalodiscus nigrescens Lankester, 1905 11.565 Harrimaniidae gen sp.
V: Smaller Coelomate Groups--Chaetognatha, Hemichordata, Pogonophora, Phoronida, Ectoprocta, Brachiopoda, Sipunculida.
Two of them are possible ancestral states in Hemichordata and Echinodermata.
Molecular phylogeny of hemichordata, with updated status of deep-sea enteropneusts.
Taxa 18S rDNA 16S rDNA HEMICHORDATA Cephalodiscidae Cephalodiscus hodgsoni KF683576 -- HI 1.3 Cephalodiscus hodgsoni EU728441 -- Cephalodiscus fumosus KF83575 -- H5.3 Cephalodiscus KF683574 -- nigrescens H12.3 Cephalodiscus gracilis AF236798 -- Cephalodiscus densus EU728439 -- Cephalodiscus EU728440 -- nigrescens Rhabdopleuridae Rhabdopleura sp.
This is the first clear instance of external brooding ever found in the phylum Hemichordata.
221-347 in Microscopical Anatomy of Invertebrates: Hemichordata, Chaetognatha and the Invertebrate Chordates, Vol.
1, Hemichordata, Chaetognatha, and the Invertebrate Chordates, G.
Nervous system of the tornaria larva (Hemichordata, Enteropneusta).
Structure of the nervous system in the tornaria larva of Balanoglossus proterogonius (Hemichordata: Enteropneusta) and its phylogenetic implications.
Structure, ultrastructure, and function of the preoral heart-kidney in Saccoglossum kowalevskii (Hemichordata: Enteropneusta) including new data on the stomochord.
The deuterostome phylum Hemichordata is composed of three extant classes: the Enteropneusta are solitary worms, the Pterobranchia are colonial tube-dwelling suspension feeders, and the Planktosphaeroidea are known only from their larvae.