Whelk

A whelk was a type of sea creature on Earth that was small enough to be manipulated by a boy. When struggling to understand their situation in the miniscope, the Third Doctor insisted that they were like sea creatures caught in a rock pool by a boy: none the wiser for the change in environment. Jo rejected the metaphor, insisting that "human beings are slightly more intelligent than whelks". (TV: Carnival of Monsters)