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Taxonomy
Though crustaceans and mollusks are parts of the animal kingdom, both of these groups belong to different phyla. A crustacean can be classified as one of the over 40,000 species of animals that fall into the phylum Crustacea, and a mollusk is any of the over 50,000 species of animals found in the phylum of Mollusca. Since an animal's phylum is based on its body plan and basic physical makeup, the fact that crustaceans and mollusks are in different phyla indicates that the animals are very different on a biological level.
Body Shape
Most mollusks feature three main body regions: a head which stores the sensory organs and the "brain," a visceral mass that stores internal organs, and a muscular "foot." Most mollusks have shells, but not all. Crustaceans, on the other hand, have a clearly segmented body, with three distinct segments: a head, thorax and abdomen. All crustaceans have a hard, protective exoskeleton made of chitin, though this exoskeleton can vary immensely in color, size and shape.
Internal Biology
Mollusks have developed bodily organs and moderately complex nervous systems, circulatory systems, and respiratory systems. Their nervous system in particular has led to some striking intelligence, most documented in octopods. A crustacean's nervous system stretches throughout the body in a nerve cord, rather than being located in the brain. The nerve cord is made up of separate ganglia, which act as "mini-brains," and control different parts of the body.
Species
Both mollusks and crustaceans have tens of thousands of species in their phyla. These species differ from each other in size, body shape, color and habitat, yet can still fall under the same phylum. Some of the best known species of mollusks include clams, oysters, snails, slugs, octopods and squids. Some of the best known species of crustaceans include lobsters, crabs, shrimp, prawns, barnacles and woodlice.