Mollusca have the largest number species of all the phylums. There are over 100,000 species known to science most of which are marine. They also show an enormous range in size, from species which are microscopic to the largest of all invertebrates, the giant squid. Most species of mollusks are covered by a shell of some sort, although a shell is not mandatory. These shells are very durable and have been collected by human beings for thousands of years. Generally mollusks have three body regions, a head, visceral mass, and a "foot". Many mollusks also have a radula which is a tongue of some sort.