- Mollusca is the second-largest phylum of invertebrate animals, after Arthropoda; members are known as molluscs or mollusks (/ˈmɒləsk/). Around 76,000 extant species of molluscs are recognized. The number of fossil species is estimated between 60,000 and 100,000 additional species. The proportion of undescribed species is very high. Many taxa remain poorly studied.