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Are simple animals with a basic body structure of two layers, padded with a gelatinous infilling called mesoglea. They belong to the phylum Coelenterata along with sea anemones (actinians) and the Portuguese man-of-war (Physalia physalis). The main body or bell is umbrella shaped and in the middle of its underside is a mouth that opens into a sacklike stomach. Muscles around the edge of the bell contract rhythmically and by jetting water propel the jellyfish through the water. Around the edge of the bell is a ring of tentacles, which can be extremely long and extensible in some of the species. Also around the mouth there is another ring of tentacles that manoeuvre food into the mouth. Both sets of tentacles carry stinging cells called nematocysts, which discharge when stimulated both by chemicals in the water and by touch. Many of the commoner species like the harmless moon jellyfish (Aurelia aurita) are filter feeders and have weak nematocysts. But the predatory species that feed on fish have nematocysts that are powerful enough to pierce human skin and are the ones that can give painful nettle-like stings. Not only are they painful, but in some species they also inject neurotoxins that in extreme cases can cause paralysis in swimmers. The most extreme example is the sea wasp (Chironex fleckeri), a type of box jellyfish which occurs seasonally around some coasts of Australia above a latitude of 26° S., mostly between October and May. Their stings can cause death within a few minutes, so take warnings about them seriously. If you are stung remember that nematocysts are triggered mechanically, so do not rub the area where you have been stung but wash it under a running tap. In Scotland, wives of trawlermen who repaired the nets used to suffer from a painful eye condition. When the nets were shaken, nematocysts from the dried tentacles of jellyfish, especially the lion's mane jellyfish (Cyanea capillata), would float around as dust in the air and get into their eyes. One rub and they would discharge, causing painful inflammation.

Other species that sting and may be found in British waters is the compass jellyfish (Chrysaora hysoscella), which has an umbrella-shaped bell with V-shaped markings and the blue jellyfish (Cyanea lamarckii), which is also umbrella shaped. The barrel or root mouth jellyfish (Rhizostoma octopus), which measures up to one metre in diameter and has a large solid bell fringed with purple, is harmless.

M. V. Angel

The life cycle of the jellyfish. It involves six stages: the adult form, or medusa, has male and female forms which sexually reproduce by releasing eggs and sperm into the water. The zygotes then transform into the next stage, planulas, which can crawl across the seabed. The planula finds a suitable site and anchors itself to the sea bottom as a polyp, a plantlike form. As the polyp grows it forms a strobila, a stack of tiny jellyfish called ephyra, that gradually break off from the strobila, a form of asexual reproduction


Subjects: Maritime History.

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