The Collaborative International Dictionary
Acalephae
Acalephae \Ac`a*le"ph[ae]\, n. pl. [NL., from Gr. ?, a nettle.] A group of C[oe]lenterata, including the Medus[ae] or jellyfishes, and hydroids; -- so called from the stinging power they possess. Sometimes called sea nettles.
Wiktionary
acalephae
n. A group of Coelenterata, including the medusas or jellyfishes, and hydroids, called sea nettles from their sting.