Mollusks

Most molluscs had a hard rock-like shell covering their soft bodies. Some, such as the whelk shown here, crawled along. Others, like bivalves, who have 2 shells surrounding them. 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.