MULLUSCA
Molluscs are the largest marine phylum (with over 85, 000 recognized species). The mantle edge secretes a shell that consists of mainly chitin and conchiolin (a protein hardened with calcium carbonate), except the outermost layer in almost all cases is all conchiolin. The mantle cavity, a fold in the mantle, encloses a significant amount of space. It is lined with epidermis, and is exposed to the habitat around it. The underside consists of a muscular foot, which has adapted to different purposes in different classes. The foot carries a pair of statocysts, which act as balance sensors for the organism. Their shell is designed as a protective layer for the organism.