One of the mysteries of the English language finally explained.
In form Mollusca, with singular or plural concord. Originally: a heterogeneous group of mostly shell-less invertebrates, forming the second order of the class Vermes in Linnaeus's classification (Systema Naturae (ed. 10, 1758) I. vi. ii. 652) and including cephalopods, echinoderms, hydroids, slugs, and annelids. Now, mainly following the classification proposed by Cuvier in 1788–1800: a phylum of invertebrates that live in aquatic or damp habitats and have a soft, unsegmented body and usually an external calcareous shell. Also, treated as plural (also mollusca): animals of this phylum (collectively or individually).
Mid 17th century; earliest use found in Robert Lovell (?1630–1690), naturalist. From post-classical Latin Mollusca, name of a group comprising cephalopods (J. Jonston Historiae Naturalis ii. 3/1, after use of classical Latin mollia (in Pliny), ancient Greek μαλάκια (in Aristotle) as the name of such a group; subsequently adopted in scientific Latin: see definition), use as noun of neuter plural of classical Latin molluscus from mollis soft + -scus, variant (with -s- extension) of -cus, suffix forming adjectives.
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