Pterobranchia
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Pterobranchia
(′ter·ə′braŋ·kē·ə)
(invertebrate zoology) A group of small or microscopic marine animals regarded as a class of the Hemichordata; all are sessile, tubicolous organisms with a U-shaped gut and three body segments.
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Pterobranchia
A group of small sessile hemichordates that may be colonial (Rhabdopleura), pseudocolonial (Cephalodiscus), or solitary (Atubaria). Each individual, or zooid, lives inside a nonchitinous tube secreted by the protosome, except for Atubaria; an aggregation of zooids is called a coenecium and can vary in shape. The protosome, or oral shield, is disciform, closes the mouth ventrally, and secretes the tube. The protocoel has symmetrical pores at the base of the first pair of arms. The mesosome, or collar, has an anteroventral mouth and one to nine pairs of dorsally ciliated tentaculated arms, which are used to collect small organisms for food. The mesocoels extend into both arms and tentacles.
The metasome is divided into a sacciform trunk and a slender ventral stalk that may be free at its end. The trunk contains the U-shaped digestive tract, with the pharynx having a single pair of gills, except in Rhabdopleura. The stomach is a sacciform expansion of the gut, and the tubular intestine curves dorsally and opens behind the arms by a middorsal anus.
The nervous system is very simple and lacks a neurochord. The buccal diverticulum is hollow, with an anteroventral heart vesicle and central sinus. The glomerulus is poorly developed, and the circulatory system is simpler that in enteropneusts. See also Hemichordata.
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Pterobranchia
| Pterobranchia | |
|---|---|
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| Rhabdopleura normani | |
Scientific classification  |
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| Kingdom: | Animalia |
| Phylum: | Hemichordata |
| Class: | Pterobranchia Lankester 1877 |
| Orders | |
Pterobranchia is a clade of small, worm-shaped animals. They belong to the hemichordata, and live in secreted tubes on the ocean floor. Pterobranchia feed by filtering plankton out of the water with the help of cilia attached to tentacles. There are about 30 known living species in the group.
The class Pterobranchia was established by Ray Lankester in 1877. It contained, at that time, the single genus Rhabdopleura. Rhabdopleura was at first regarded as an aberrant polyzoon, but with the publication of the Challenger report on Cephalodiscus in 1887, it became clear that Cephalodiscus, the second genus now included in the order, had affinities in the direction of the Enteropneusta.
Studies under an electron microscope have suggested that pterobranchs belong to the same clade as the extinct graptolites.[1]
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Contents
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Biology
Pterobranchs are small, worm-like animals, living on the ocean floor, often in relatively deep waters. Like their relatives, the acorn worms, their body is divided into three parts: an anterior proboscis, a collar, and a trunk. The proboscis is wide and flattened at the tip, and in most species contains glands that secrete a tube of organic material within which the pterobranch spends its adult life. The animals are colonial, with several zooids living together in a cluster of tubes. In some species, the individual zooids within the colony are connected by stolons. The genus Atubaria is unusual in lacking the tubes typical of other pterobranchs.[2]
The collar bears a number of large arms, each of which includes a row tentacles along one side. The number of arms varies between species, with anything from one to nine pairs being present. The tentacles are covered in cilia and aid in filtering food from the water. The trunk includes a simple tubular gut, and is curved over so that the anus projects upwards, lying dorsal to the collar. Cephalodiscus has a single pair of gill slits in the pharynx, although Rhabdopleura has none.[2]
Pterobranchs are dioecious, with the fertilised egg hatching to produce a free-swimming ciliated larva. Despite the close relationship between the two groups, the larva does not resemble that of the acorn worms. Eventually, the larva settles onto the substrate, and reproduces asexually by budding to create a new colony.[2]
Taxonomy
The class is a small one, with only three known families, each containing a single genus.
Class Pterobranchia
- Order Cephalodiscida
- Family Atubaridae
- Genus Atubaria
- Species Atubaria heterolopha Sato, 1935
- Genus Atubaria
- Family Cephalodiscidae
- Genus Cephalodiscus
- Cephalodiscus agglutinans Harmer & Ridewood, 1914
- Cephalodiscus atlanticus Bayer, 1962
- Cephalodiscus australiensis author unknown
- Cephalodiscus calciformis Emig, 1977
- Cephalodiscus densus Andersson 1907
- Cephalodiscus dodecalophus McIntosh 1882
- Cephalodiscus evansi Ridewood
- Cephalodiscus fumosus John, 1932
- Cephalodiscus gilchristi Ridewood, 1908
- Cephalodiscus gracilis Harmer 1905
- Cephalodiscus graptolitoides Dilly 1993
- Cephalodiscus hodgsoni Ridewood, 1907
- Cephalodiscus indicus Schepotieff 1909
- Cephalodiscus kempi John, 1932
- Cephalodiscus levinsoni Harmer
- Cephalodiscus nigrescens Lankester, 1905
- Cephalodiscus sibogae Harmer 1905
- Cephalodiscus solidus Andersson, 1907
- Genus Cephalodiscus
- Family Atubaridae
- Order Rhabdopleurida
- Family Rhabdopleuridae
- Genus Rhabdopleura
- Rhabdopleura annulata Norman, 1921
- Rhabdopleura compacta Hincks, 1880
- Rhabdopleura grimaldi Julien
- Rhabdopleura manubialis Julien 1903
- Rhabdopleura normani Allmann, 1869
- Rhabdopleura striata Schepotieff, 1909
- Genus Rhabdopleura
- Family Rhabdopleuridae
Evolution
A 2011 paper in Current Biology describes an early pterobranch. The 525-million-year-old fossil from China is named Galeaplumosus abilus.[3][4]
Footnotes
- ^ Fortey, Richard A. (1998). Life: a natural history of the first four billion years of life on earth. New York: Alfred A. Knopf. p. 129. ISBN 0-375-40119-9.
- ^ a b c Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 1026–1027. ISBN 0-03-056747-5.
- ^ Remarkable Fossil: 525-Million-Year-Old Discovery of 'Feathered Helmet from Beyond the Clouds'
- ^ Xian-guang Hou, Richard J. Aldridge, David J. Siveter, Derek J. Siveter, Mark Williams, Jan Zalasiewicz, Xiao-ya Ma. A pterobranch hemichordate zooid from the lower Cambrian. Current Biology, 24 March 2011 DOI: 10.1016/j.cub.2011.03.005
References
- Halanych KM. (1995). "The Phylogenetic Position of the Pterobranch Hemichordates Based on 18S rDNA Sequence Data". Molecular phylogenetics and Evolution 4 (1): 72–76. doi:10.1006/mpev.1995.1007. PMID 7620637.
This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press.- Cameron, C. B. Classification of the extant hemichordata
- At Billie Swalla's site
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