Phylum Mollusca
Mollusks are the second most successful phylum, right behind anthropods. There are currently about 110,000 documented species that fall under Phylum Mollusca. Most mollusks live in the ocean or in an underwater habitat. Since most mollusks have shells, there is a vast fossil record of them. Mollusks are believed to have evolved from an ancestor of the flatworm about 650 million years ago. Some characteristics that most mollusks share are that they usually have an open circulatory system, they are usually bilaterally symmetrical, and they usually possess a fairly developed digestive system with a mouth, gut, and anus.