Calder, Dale R. Department of Natural History, Royal Ontario Museum, Ontario, Canada.
- Defining characteristics
- Reproduction and development
- Fossil record and evolution
- Links to Primary Literature
- Additional Readings
A vernacular name applied to plantlike polypoid stages of certain hydrozoan animals (phylum Cnidaria). Hydroids are predominantly bottom-dwelling, sedentary, colonial, and polymorphic. Two major groups are recognized: Anthoathecata (Fig. 1) and Leptothecata (Fig. 2). Leptothecate hydroids (for example, Laomedea, Obelia) usually possess chitinous hydrothecae, gonothecae, and nematothecae, which are exoskeletal structures that protect feeding polyps (hydranths), reproductive polyps (gonozooids), and defensive polyps (nematophores), respectively. Anthoathecate hydroids (for example, Hydractinia, Pennaria) lack hydrothecae, gonothecae, and nematothecae. Closely related to hydroids are the largely holopelagic siphonophores. Approximately 3000 species are recognized worldwide, with most being marine. See also: Cnidaria; Deep-sea fauna; Hydrozoa
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