Definitions for trachymedusae
a division of acalephs in which the development is direct from the eggs, without a hydroid stage. Some of the species are parasitic on other medusae
Trachymedusae are an order in the cnidarian class Hydrozoa. They contain some 50 species, divided among about 30 genera and 5 families, of which the Rhopalonematidae are by far the most diverse. They only reproduce sexually as medusa, never forming polyps. It is not known with certainty whether the group as delimited here is indeed monophyletic. Predominantly living in the deep open oceans, they are small medusae with hemispherical to heightened bells that have a smooth and straight margin of thick tissue with ample nematocysts. They have 4, 6, 8 or more radial canals, with 8 being most common. The gonads are borne at the radial canals or where these meet the manubrium. The statocysts are of mixed tissue from both endoderm and ectoderm. Their cnidome can include stenoteles. Families: ⁕Geryoniidae ⁕Halicreatidae ⁕Petasidae ⁕Ptychogastriidae ⁕Rhopalonematidae
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