Body Plan

Mollusks have a thin layer of tissue called the mantle. The mantle covers the body organs, which are located in the visceral mass. Between the soft body and the mantle is a space called the mantle cavity. Mollusks contain gills. Gills are organs in which carbon dioxide from the mollusk is exchanged for oxygen in the water. 

The mantle also protects the body if the mollusk does not have a shell. the shell is made up of several layers. The inside layer is the smoothest but it is usually the thickest layer because it's added to throughout the life of the mollusk. The inside layer protects the soft body. 

The circulatory system of most mollusks is an open system. In an open circulatory system, the heart moves blood out into the open spaces around the body organs. The blood contains nutrients and oxygen. the blood completely surrounds and nourishes the body organs.

Most Mollusks have a well-developed head with a mouth and some sensory organs. Some Mollusks, such as the squid, have tentacles. The muscular foot, which is used for movement, is in the underside of the mollusk.