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A slug trades the shell for mobility , energy saving in not building a shell , and no need for calcium salts . We appreciate notification of any errors or inconsistencies in the data . Is your specimen in the correct proportion to be a juvenile ? Small relatives of the operculate genus Colobostylus can move 7 times their shell length each minute . In addition to his affiliation with the Academy , he is a Scientist Emeritus with the U.S. Geological Survey .