Mollusk

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Most molluscs had a hard rock-like shell covering their soft bodies. Some, such as the whelk shown here, crawled along. Others, like oysters, had a hinged shell which opened like a box. Yet others lost their hard shell, like the octopus. To get enough energy to move, molluscs and most other animals have an organ called a gill. This absorbs oxygen from the water and gives out carbon dioxide.

Molluscs were one of the most successful of all animal groups.