It was discovered 24 years ago in a small, salt-water lagoon at Shoreham, on the West Sussex seaside, by Richard Ivell, a graduate zoology student at Oxford University. It was quite new to science, and was given the official name Edwardsia ivelli. To have a creature named after you is a great coup for any biologist, even more so for a young one.
Mr Ivell's find was only three millimetres long, and almost colourless. It spent most of its life burrowing in the uppermost layer of mud in the lagoon; it had nine tentacles, each about six millimetres long, which it spread out through the ooze in search of food.
The creature was only ever found in that one location on a few occasions. When the lagoon, cut off from the sea by a shingle bank, was thoroughly searched a few years later, in 1983, the anemone - a fairly simple kind of animal - had disappeared.
Last month, a marine biology consultant carried out a new and exhaustive search, commissioned by the World Wide Fund for Nature. He too failed to find the anemone and so the fund is pronouncing the species extinct. All that remains are a few pickled specimens in little jars, kept at the Oxford Museum.
''This is a sad day for British conservation,'' said Callum Rankine, of WWF-UK. ''It does not bode well for the future to lose such a rare species.'' He speculates that changes in salinity and temperature in the lagoon were to blame. The sea anemone was one of 116 very rare or declining British plant and animal species covered by "action plans" agreed between the Government and the nation's leading wildlife conservation bodies, such as WWF. The action plan for this species consisted mainly of trying to find it. The man who formally introduced the species to the scientific world is Richard Manuel, a technician at Oxford University who wrote a paper describing the anemone in great detail in the Journal of Natural History. And he believes it may not be extinct. ''Perhaps it lives in other countries, but never has been found and described. There are saline lagoons on the other side of the Channel in France with crystal clear water - perhaps it lives there,'' he said. Mr Ivell, by the way, now lives in Germany.
The last species unique to Britain to enter the eternal night of extinction was a grass, Bromus interruptus. But not quite. It only actually died out in the wild in 1972, but lives on a botanical garden.
But even as we lose one species, we gain another. For nature conservationists, yesterday's other big news was the announcement that the little egret (above) has begun breeding in Britain. Global warming may be the explanation.
It is a small, snowy white heron which lives in the Middle East and along the shores of the Mediterranean. But this sub-tropical species has gradually been moving north. Before the war it was unknown in Britain, but its numbers have been creeping up as it flies over from northern France. Several hundred can be present along the south coast of England at the same time. The question was - when would they become breeding residents? The Wildlife Trusts, a nationwide coalition of county conservation groups, said it had proof that the egrets have now nested and successfully raised young for two years at two sites along the Dorset Coast, one of them Brownsea Island in Poole Harbour.