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 when the Challenger report on Cephalodiscus was published 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, and phylogenetic analysis suggests that the pterobranchs are living members of the graptolite clade.
Pterobranchs are small worm-like filter-feeders, 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 in which the pterobranch spends its adult life. The animals are mostly 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 single member in the genus Atubaria is unusual in lacking the tubes typical of other pterobranchs, but is regarded as a questionable species and is no longer considered valid.
The collar bears a number of large arms, each of which includes a row of tentacles along one side. The number of arms varies between species, with anything from one to nine pairs. 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.
Development of pterobranchs have been studied only in Rhabdopleura from Plymouth (R. compacta)  and from Bermuda (R. normani). Both of these species 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; they are "planula-like". Eventually, the larva settles onto the substrate and metamorphoses to an adult. Alternatively, they also reproduce asexually by budding to create a new colony.
The class is a small one, with only three known families, each containing a single genus.
- Order Cephalodiscida
- Family Atubaridae
- 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
- Order Rhabdopleurida
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- 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). "Pterobranchia". Encyclopædia Britannica (11th ed.). Cambridge University Press.
- Cameron, C. B. Classification of the extant hemichordata
- At Billie Swalla's site