<P> Molluscs (Latin for soft) form a phylum with about 85,000 extant recognized species . They are the largest marine phylum in terms of species count, comprising about 23% of all the named marine organisms . Molluscs have more varied forms than other invertebrate phylums . They are highly diverse, not just in size and in anatomical structure, but also in behaviour and in habitat . The majority of species still live in the oceans, from the seashores to the abyssal zone, but some form a significant part of the freshwater fauna and the terrestrial ecosystems . </P> <P> The mollusc phylum is divided into 9 or 10 taxonomic classes, two of which are extinct . These classes include gastropods, bivalves and cephalopods, as well as other lesser - known but distinctive classes . Gastropods with protective shells are referred to as snails, whereas gastropods without protective shells are referred to as slugs . Gastropods are by far the most numerous molluscs in terms of classified species, accounting for 80% of the total . Bivalves include clams, oysters, cockles, mussels, scallops, and numerous other families . There are about 8,000 marine bivalves species (including brackish water and estuarine species), and about 1,200 freshwater species . Cephalopod include octopus, squid and cuttlefish . They are found in all oceans, and neurologically are the most advanced of the invertebrates . About 800 living species of marine cephalopods have been identified, and an estimated 11,000 extinct taxa have been described . There are no fully freshwater cephalopods . </P> <Ul> <Li> <P> Colossal squid, largest of all invertebrates </P> </Li> <Li> <P> The nautilus is a living fossil little changed since it evolved 500 million years ago as one of the first cephalopods . </P> </Li> <Li> <P> Marine gastropods are sea snails or sea slugs . This nudibranch is a sea slug . </P> </Li> <Li> <P> The sea snail Syrinx aruanus has the largest shell of any living gastropod </P> </Li> <Li> <P> Molluscs usually have eyes . Bordering the edge of the mantle of a scallop, a bivalve mollusc, can be over 100 simple eyes . </P> </Li> <Li> <P> Common mussel, another bivalve </P> </Li> </Ul> <Li> <P> Colossal squid, largest of all invertebrates </P> </Li>

Where do jelly fish spend the majority of their life