The molluscs or mollusks (pronounced /ˈmɒləsks/), compose a large phylum ofinvertebrate animals, Mollusca. Around 85,000 extant species of molluscs are recognized. Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrialhabitats. They are highly diverse, not only in size and in anatomical structure, but also in behaviour and in habitat.The phylum is typically divided into 9 or 10taxonomic classes, of which two are entirely extinct. Cephalopod molluscs, such assquid, cuttlefish and octopus, are among the most neurologically advanced of allinvertebrates—and either the giant squid or the colossal squid is the largest known invertebrate species. The gastropods (snails and slugs) are by far the most numerous molluscs in terms of classified species, and account for 80% of the total.