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Adult S. balanoides grow up to 15 millimetres (0.6 in) in diameter, and are sessile, living attached to rocks and other solid substrates. They have six greyish wall plates surrounding a diamond-shaped operculum. The base of the shell is membranous in Semibalanus, unlike other barnacles which have calcified bases. When the tide rises to cover the barnacles, the operculum opens, and feathery cirri (modified thoracic appendages) are extended into the water to filter food from the seawater. When the tide falls, the operculum closes again to prevent desiccation the reduction from the primitive condition of eight wall plates to six is believed to decrease water loss even further by reducing the number of sutures through which water can escape. In Europe, S. balanoides is found on Svalbard and from Finnmark to north-west Spain but excluding part of the Bay of Biscay. It is common throughout the British Isles, except in parts of Cornwall, the Scilly Isles and south-western Ireland. On the North American coast of the Atlantic Ocean, it reaches as far south as Cape Hatteras, North Carolina, and on the Pacific coast, it reaches as far south as British Columbia. S. balanoides is the most common and widespread intertidal barnacle in the British Isles, and the only intertidal barnacle of the north-east coast of North America.
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