Halanych, Kenneth M. Department of Biological Sciences, Auburn University, Auburn, Alabama.
Cannon, Johanna T. Department of Biological Sciences, Auburn University, Auburn, Alabama.
Benito, Jesús Department of Animal Biology I-Zoology of Invertebrates, University Complutense, Madrid, Spain.
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A group of sessile hemichordates that is colonial with small individuals, or zooids, typically 3–6 mm (0.12–0.24 in.) in length. The group Pterobranchia is allied with acorn worms, or enteropneust hemichordates, and there is some question as to whether pterobranchs evolved from within acorn worms. Pterobranchs are suspension feeders that typically live in high-flow regions and feed on algal particles and suspended organic matter. Most species occur in deep cold water, but shallow tropical species are known. Possibly allied to the extinct graptolites, there are approximately 24 living species, divided into two genera. Most of these species live in the Southern Hemisphere. Members of the genus Rhabdopleura (6 species) are distinguished by each individual being housed in its own erect coenecium, a nonchitinous tube secreted by the cephalic shield (Fig. 1). Inside the creeping tube, a tissue process (stolon) forms and connects all the zooids of the colony by budding. In contrast, members of the genus Cephalodiscus (18 species) live in a more open tube, with clusters of individuals sharing a common disklike structure at the end of a flexible trunk (Fig. 2). Some colonies can obtain large sizes [40 cm (16 in.)] while maintaining a series of more open tubes. The cluster consists of several asexually budded individuals that can move as a unit or, presumably, undergo fission to continue asexual reproduction. For both genera, the tube structure, or coenecium, can vary in shape from species to species; it also shows variation in relation to environmental conditions. See also: Enteropneusta; Graptolithina; Hemichordata
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