The exoskeleton is an external skeleton formed by a hard enclosure on an organism's surface. Crabs and insect shells, for example, are exoskeletons. This type of skeleton provides defence against predators, supports the body, and allows movement through muscle contraction. Like vertebrates, inside the exoskeleton muscles must cross a joint. Muscle shortening changes the relationship between the exoskeleton's two segments. Arthropods like crabs and lobsters have exoskeletons consisting of 30% to 50% chitin, a polysaccharide glucose derivative that is a strong but flexible material. The epidermal cells secrete chitin. The exoskeleton is further reinforced in organisms such as the lobster by adding calcium carbonate. Since the exoskeleton is acellular, arthropods need to shed their exoskeletons periodically because the exoskeleton does not grow as the organism grows.
Let's divide snails' body into the shell and the soft body that holds it to analyze the snails ' external anatomy. The former is a solid spiral-shaped structure carried on the back, consisting mainly of calcium carbonate, made of a single piece. The shell's central layer, called ostracum, has two layers of the same substance's crystals, calcium carbonate. The Hipostracum is below and the most superficial layer is the periostracum, which is made up of many proteins.
Inside, there are no divisions in the snails ' body. The internal organs, including gonads, intestines, heart and esophagus create a mantle-protected organic mass. They are pulmonary animals, meaning they have a lung that is specialized in using the oxygen obtained from atmospheric air breathing.