DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Obelia medusa. Light micrograph of a single specimen of the medusa stage of a colonial marine invertebrate, the sea fir Obelia geniculata. Similar in appearance to a jellyfish, this is the free-swimming stage, released from the sessile, stalked, colonial polyp stage. Each medusa has a ring of solid tentacles all with a swelling at their base. Within the medusa are structures such as statocysts, canals and gonads, all grouped symmetrically (four of these groupings are seen here at right angles). The central structure (round) is called the manubrium, and has oral tentacles. Magnification: x7 when printed at 10 centimetres wide.
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