Crowell, Sears Department of Zoology, Indiana University, Bloomington, Indiana.
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A subclass of jellyfish of the class Hydrozoa of the phylum Cnidaria. Most members of the subclass Trachylina (also spelled Trachylinae and collectively called trachylines) are jellyfish of moderate size [see illustration]. They differ from other hydrozoan jellyfish in having balancing organs that develop partly from the digestive epithelium and in having only a small polyp stage or none at all. In older taxonomies, Trachylina was classified as an order. Authorities now recognize Trachylina as a subclass, comprising four distinct orders: Actinulida, Limnomedusae, Narcomedusae, and Trachymedusae. See also: Cnidaria; Hydrozoa
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