Mollusks are a phylum of invertebrate animals. Around 85,000 extant species of mollusks are recognized. Mollusks are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous mollusks also live in freshwater and terrestrial habitats. They are highly diverse, not just in size and in anatomical structure, but also in behaviour and in habitat. The phylum is typically divided into 9 or 10 taxonomic classes, of which 2 are entirely extinct. Cephalopods (squid, cuttlefish and octopus) are among the most neurologically advanced of all invertebrates—and either the giant squid or the colossal squid is the largest known invertebrate species. Gastropods (snails and slugs) are by far the most numerous mollusks in terms of classified species, and account for 80% of the total.