This section is from the book "The Home Cyclopedia Of General Information", by Charles Morris. Also available from Amazon: Home Cyclopedia of Necessary Knowledge.
A genus of ocean animals, called also acorn shells. These belong to the order of the crustaceae, swim when young, but afterwards attach themselves to rocks, ships, sea plants, etc., develop a shell, and become fixed for life. They gather thickly on the bottoms of vessels in the tropics and much impede their speed.
 
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