Mollusks are the oldest creatures in the world and their most characteristic feature is that they do not have an internal and external skeleton, so they are called "invertebrates". Scientists divide them into families and some of them are called: the "gastropods" (barefoot, snails). the "bivalve molluscs" (mussels, oysters) and the "cephalopods" (octopuses and cuttlefish). Gastropods are evolved molluscs. Their special feature is their large foot, which they use to move or to dig. The bivalve mollusks form a hard outer skeleton, called a "limestone shell". Such are mussels, oysters and clams. Finally, the cephalopods breathe from their mantle and use it to avoid their enemies by spewing ink, making the animals flee in the opposite direction. They use their tentacles to feed, walk, and reproduce. They have suction cups on their tentacles that they use like human fingers.