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Order Trachymedusae Haeckel, 1879 |
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diagnosis |
Medusa bell hemispherical to higher than broad, margin not lobed, with ring of thickened tissue studded with nematocysts, mostly with 8 radial canals, 4, 6 or more than 8 also possible, gonads on radial canals or at junction of radial canals and manubrium, statocysts of endo-ectodermal origin. Polyp stage absent, development direct. Cnidome can include stenoteles. Predominantly forms of the open sea and deeper waters. |
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higher classification |
Cnidaria, Medusozoa, Hydrozoa, Trachylinae |
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species |
about 53 |
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how to recognize? |
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Trachymedusae are sometimes not easy to separate from Limnomedusae. Eight radial canals, a ring of thickened tissue studded with nematocysts, and the gonads on radial canals are usually the best indicators that you have a Trachymedusa. The small number of Trachymedusae with less than eight tentacles makes it relatively easy to recognize these medusae. See the overview on the families below. |
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pendant statocyst |
A longitudinal section of pendant trachymedusan statocyst, after electron-optic images, the stiff sensory cilia serve to sense the motion of the organ, note that the core is derived from entodermal tissue (green). The whole structure can be homologized to a modified tentacle. See also a photo of an internal statocyst. |
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Families |
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Trachymedusae with gastric peduncle; usually four radial canals, sometimes more; with centripetal canals; gonads flat cushions on radial canals; two kinds of marginal tentacles, solid and hollow; statocysts enclosed in mesogloea. |
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Trachymedusae with a small manubrium; with or without peduncle; no centripetal canals; usually eight radial canals, rarely more, radial canals narrow; mouth with distinct lips; marginal tentacles evenly distributed, sometimes two types of tentacles, tentacle of uniform structure or with proximal portion differing from distal one; gonads either on radial canals, or forming a continuous ring around base of manubrium and extending outwards along radial canals; statocysts with endodermal core either free and pendant, or rarely enclosed by an exumbrellar outgrowth. |
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Trachymedusae with wide, circular manubrium; mouth circular, without distinct lips; without peduncle; without centripetal canals; usually with eight radial canals (except Varitentaculata which has four), radial canals broad; numerous marginal tentacles of different size, but all structurally alike and arranged in a single series; each marginal tentacle with flexible proximal portion and stiff distal portion; with pendant statocysts. |
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Trachymedusae with four radial canals; no peduncle and centripetal canals; manubrium well developed; four sac-like gonads on radial canals; marginal tentacles not in clusters, solid, with a terminal club-shaped knob of cnidocysts; pendant statocysts. |
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Trachymedusae with either simple manubrium without mesenteries, or with eight-lobed manubrium fixed by eight mesenteries to subumbrella; with either marginal tentacles grouped into more or less well defined clusters, some with adhesive disks, or with very numerous tentacles, not in clusters, but inserted at various levels on exumbrella; eight radialcanals, no centripetal canals or peduncle; with eight radial canals; gonads either attached onto manubrium on sides of the eight manubrial lobes, or on radial canals adjacent to manubrium; pendant statocysts. Bottom dwelling forms. |
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Remarks |
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Only few Trachymedusae will be encountered close to continents. They are predominantly forms of the open ocean and of deep waters. |
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References |
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Russell, F. S. 1953.
The medusae of the British Isles. Cambridge University Press,
London, pp. 530, 35 pls. |
this page is part of the Hydrozoa Directory ©Peter Schuchert January 2005