The Mollusca (mollusks) are a large phylum of animals that includes the snails, slugs, clams, squids, and octopi, among others. Most are marine, many are freshwater, and some snails and slugs are terrestrial. The phylum name refers to their soft, pulpy bodies ( mollis means "soft"). In many cases, the body is protected by a hard shell of calcium carbonate—the seashells familiar to beachcombers and "half shells" familiar to oyster lovers.
The shell is secreted by a membrane called the mantle that envelops the body like a cloak. In species without an external shell, such as octopi, the mantle forms an outermost skinlike body covering. The mantle encloses a space, the mantle cavity, which usually contains comblike gills for respiration. In some seemingly shell-less species—squids and cuttlefish—the shell is embedded in the mantle and can be found only by dissection. Most mollusks also have a radula. In snails, this is a tonguelike belt equipped with a few hundred to thousands of chitinous teeth, used to scrape food from surfaces such as rocks.
The most behaviorally sophisticated of all invertebrate animals are the cephalopod mollusks: the octopi, squids, cuttlefish, and nautilus. Cephalopods have long, flexible arms, equipped in most cases with suckers for prey capture. They are active swimmers; some have eyes remarkably similar to human eyes; they have more complex brains than any other invertebrate; and, correspondingly, they exhibit remarkably subtle social behaviors and learning capabilities.
Kenneth S. Saladin
Pechenik, Jan A. Biology of the Invertebrates, 4th ed. Boston: McGraw-Hill, 2000.
Rupert, E. E., and R. D. Barnes. Invertebrate Zoology, 6th ed. Philadelphia, PA: W. B. Saunders, Co., 1994.