Arthropods are the larger of the two groups of invertebrates, and mollusks are the second largest. Invertebrates are animals without backbones. Arthropods have a hard exoskeleton, made out of chitin, while mollusks have shells for a protective covering. They both have a muscular interior, to facilitate movement. Both have a digestive system, with mouths, and other sense organs. Both can be found in fresh water and salt water. Both are sexually reproductive, requiring male and female to reproduce. Both seem to go through developmental stages, through a process of metamorphosis, to achieve adulthood. Arthropods include lobsters, shrimp, cuttlefish, and the like. Mollusks include, oysters, clams, snails, and other shell-bearing animals. Between the both of them, arthropods and mollusks make up a larger percentage of the invertebrate animal population.