DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Obelia polyps. Light micrograph of polyps on a stalk from a colonial marine invertebrate, the sea fir Obelia geniculata. The upper parts of the stalk consist of cup-shaped feeding polyps (gastrozoids). The base has both polyps and reproductive structures called gonothecae. The feeding polyps have a ring of tentacles with stinging hairs (cnidiocysts) that paralyse the prey which is then drawn into the polyp and digested. The gonothecae are the site of asexual reproduction, where medusae bud off to drift in the water and form new colonies. Obelia is commonly found on Fucus seaweed. Magnification: x5 when printed at 10 centimetres tall.
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