acalephæ

from The Century Dictionary.

• A name given to a large number of marine animals included in the subkingdom Cælenterata, and represented chiefly by the Medusidæ and their allies, in popular language known as sea-nettles, sea-blubbers, jelly-fish, etc.
• A clans of Cælenterata, including medusæ of considerable size, with gastral filaments, endodermal gonads, and lobed umbrella-edge, and without true velum: contrasted with Hydromedusæ and Actinozoa. Same as Acraspeda.

from the GNU version of the Collaborative International Dictionary of English.

• noun plural A group of Cœlenterata, including the Medusæ or jellyfishes, and hydroids; -- so called from the stinging power they possess. Sometimes called sea nettles.