The molluscs or mollusks /ˈmɒləsks/ compose the large phylum of invertebrate animals known as the 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 terrestrial habitats.round 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 terrestrial habitats.