Mollusca
The Mollusca, common name molluscs or mollusks, are a large phylum of invertebrate animals. There are around 85,000 recognized species of molluscs. Mollusca is the largest marine phylum, comprising about 23% of all the named marine organisms. Many molluscs also live in freshwater and terrestrial habitats. Molluscs are highly diverse, not only in size and in structure, but also in behavior and in habitat.
Nautiloids
This organism to the left is an Nautiloid. Nautiloids are a large and diverse group of marine cephalopods belonging to the subclass Nautiloidea that began in the late Cambrian era. It is a species of Mollusca that lives in the ocean. It has a shell and it swims or floats around. The shell of the Nautilus is basically its skeleton. Their shell also serves as a protector from prey. It also helps maneuver them around the ocean floor.
Snails
The gastropods (snails and slugs) are by far the most numerous molluscs in terms of classified species, and account for a large amount of the Molluscan population. Like most Mollusks the shell on the back of the snail serve as the skeleton for it, as well as for protection and camoflouge. The gastropod shell is a shell which is part of the body of a gastropod or snail, one kind of mollusc. The gastropod shell is an external skeleton or exoskeleton, which serves not only for muscle attachment, but also for protection from predators and from mechanical damage. In land snails, and in some freshwater snails and in marine snails, the shell is also an essential protection against the sun, and against drying out. Most gastropod shells are spirally and the coiling is usually right-handed. Some gastropods are shell-less (slugs), but the majority of gastropods do have a shell. In almost every case the shell consists of one piece, and is typically spirally coiled, although some groups, such as the various families and genera of limpets, have simple cone-shaped shells as adults.
Clams
A bivalve shell is part of the body, the exoskeleton or shell, of a bivalve mollusk. In life, the shell of this class of mollusks is composed of two parts, two valves which are hinged together. Bivalves are very common in many kinds of saltwater habitats, but they are also found in brackish water and in freshwater. Bivalves are a consistant part of the marine life worldwide (scallops, clams, oysters, mussels, etc). The shells of marine bivalves commonly wash up on beaches (often as separate valves) and the shells of freshwater species can sometimes be discovered along the flood plains of rivers, and other freshwater habitats.This exoskeleton serves not only for muscle attachment, but also for protection from predators and from mechanical damage. The shell has many layers, and is typically made of calcium carbonate made out into an organic matrix. It is extracted by a part of the molluscan body known as the mantle.