Jelly-fish

Jelly-fish (Medusæ), bell-shaped or disc-like marine Hydrozoa, for the most part active swimmers. One set, known as Acraspeda or Acalephæ, are usually large, with a climax in a giant specimen of Cyanea, which had a bell 7½ feet across, and tentacles 120 feet long. Beset with myriads of stinging cells, these 'blubbers' often make bathers more than uncomfortable. They are frequently left stranded in great numbers on the beach by the retiring tide. The common Aurelia is a well-known representative, while the exceptional Lucernarians are noteworthy in leading a more or less sedentary life attached to seaweeds and other objects. Anatomically different from the above, and included among the Craspedote Hydrozoa (q.v.), are the Trachymedusæ, of which Geryonia is a good type. Finally, a great number of Medusoid forms, usually small in size, very closely resemble the Trachymedusæ, but differ both from them and from the Acraspeda in being the liberated sexual 'persons' of Hydroid or Zoophyte colonies. See CELENTERATA; GENERATIONS (ALTERNATION OF); and for exact classification, HYDROZOA.