word-forming element denoting orders and classes in zoology, from Latin -acea, neuter plural of -aceus "belonging to, of the nature of" (enlarged from adjectival suffix -ax, genitive -acis); neuter plural because of a presumed animalia, a neuter plural noun. Thus, crustacea "shellfish" are *crustacea animalia "crusty animals." In botany, the suffix is -aceae, from the fem. plural of -aceus, with reference to Latin plantae, which is a fem. plural.