Malacology is the branch of invertebrate zoology that deals with the study of the Mollusca (mollusks or molluscs), the second-largest phylum of animals in terms of described species after the arthropods. Mollusks include snails and slugs, clams, octopus and squid, and numerous other kinds, many (but by no means all) of which have shells. One division of malacology, conchology, is devoted to the study of mollusk shells. Malacology derives from Greek , malakos, "soft"; and , -logia.