One of the mysteries of the English language finally explained.
Treated as plural. A former class of invertebrates lacking distinct heads, broadly equivalent to Bivalvia (bivalve molluscs) but also including tunicates; (also in form acephala) invertebrates of this class collectively. Occasionally with singular concord (also in form acephala): an invertebrate of this class.
Early 19th century. From scientific Latin Acephala, plural noun, use as noun of post-classical Latin acephalus, after French acéphales, plural noun; compare scientific Latin -a.
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