The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]