Department of Zoology at ANDC/Zoology Museum/Museum specimens/Cnidaria/Obelia

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Date & Time : 29, November 2023 17:31

Obelia geniculata


Phylum - Cnidaria

Class - Hydrozoa

Order - Hydroida

Genus - Obelia

Common Name

Sea Fur

Geographical Distribution

It is found from Arctic region to the Gulf of Mexico and the Pacific coast, and from Southern California to Oregon.


It is colonial, marine, sedentary hydrozoan zoophyte found attached on piles, rocks and sea weeds in shallow water.

Identifying Features

Labelled diagram of OBELIA.jpg

  • It is a colonial hydroid.
  • The colony consists of basal horizontal portion, the hydrorhiza which is attached with the substratum and number of vertical branches known as hydrocauli arising from hydrorhiza.
  • The colony is trimorphic having three types of zooids, e.g., hydranth, blastostyle and medusa.
  • Hydranth or polyp has a cylindrical body attached to the axis of the hydrocaulus by its proximal end free at its distal end.
  • It is covered by cup-shaped hydrotheca. It is nutritive zooid of colony.
  • The hypostome is surrounded by a number of solid tentacles provided with nematocytes.
  • Blastostyle or reproductive zooid is cup-shaped without mouth and tentacles.
  • It is enclosed by a covering, gonotheca.
  • It gives rise to buds which later become flattened and develope into new medusae.
  • Medusa is bell-shaped with a concave and convex side. It contains marginal tentacles, four radial canals, a ring canal, four gonads born on the radial canals and hanging central manubrium on the concave side.
  • The medusae are free swimming.
  • Life history of Obelia exhibit an alternation of generation, called metagenesis.