The word snail is a common name for almost all members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word snail is used in a general sense, it includes sea snails, land snails and freshwater snails. Snails lacking a shell or having only a very small one are usually called slugs. Snails that have a broadly conical shell that is not coiled or appears not to be coiled are often known as limpets.common namemolluscan Gastropodashellsslugs limpets

Slug is a common non-scientific word, which is often applied to any gastropod mollusk whatsoever that has a very reduced shell, a small internal shell, or no shell at all. Gastropods with coiled shells that are big enough to retract into are called snails. Land gastropods with a shell that is not quite vestigial, but is too small to retract into, like many in the family Urocyclidae, are known as "semislugs".gastropod mollusksnailsvestigialUrocyclidae

The common name mussel is used for members of several different families of clams or bivalve molluscs, from both saltwater and freshwater habitats. The one thing that these different groups have in common, is that they have a shell whose outline is somewhat elongated and asymmetrical compared with that of many other edible clams, the shells of which are often more or less rounded or oval in shape.common name clamsbivalvemolluscs

The common name oyster is used for a number of different groups of bivalve mollusks, most of which live in marine habitats or brackish water. The shell consists of two usually highly calcified valves which surround a soft body. Gills filter plankton from the water, and strong adductor muscles are used to hold the shell closed.bivalve mollusksbrackish waterfilterplanktonadductor

Squid are marine cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms and two tentacles arranged in pairs. (The only known exception is the bigfin squid group, which have ten very long, thin arms of equal length.)marinecephalopodsorderbilateral symmetry mantlearmscuttlefisharmstentaclesbigfin squid

The octopus (pronounced / ˈɒ kt ə p ə s/, from Greek ὀ κτάπους (oktapous), "eight-footed", [1][2] with plural forms: octopuses [ ˈɒ kt ə p ʊ s ɪ z], octopi [ ˈɒ kt ə pa ɪ ], or octopodes [ ˌɒ k ˈ t ə ʊ p ə di ː z], see below) is a cephalopod of the order Octopoda that inhabits many diverse regions of the ocean, especially coral reefs. The term may also refer to only those creatures in the genus Octopus. In the larger sense, there are around 300 recognized octopus species, which is over one-third of the total number of known cephalopod species/ ˈɒ kt ə p ə s/Greek [1][2]belowcephalopod orderoceancoral reefsgenusOctopusspecies