Jellyfish (also called jellies or sea jellies as they
are not true fish) are animals that belong to Phylum Cnidaria,
included in the class Scyphozoa (from Greek skyphos "cup"
and zoon "animal"). The name "jellyfish"
is also sometimes used for the related classes of medusae
(Hydrozoa) and box jellyfish (Cubozoa). Almost all jellyfish
live in the seas and though they lack true organ structures
they feature specialized tissues. The adult forms of these
creatures are composed of 95-99% water. All species are
found in each of the world's oceans, with a few species
living in fresh water. Most jellyfish are passive drifters
that feed on small fish and zooplankton that become caught
in their tentacles. Jellyfish have an incomplete digestive
system, meaning that the same orifice is used for both
food intake and waste expulsion. They are Coelenterates
which means "hollow gut", and are made up of
a layer of epidermis, gastrodermis, and a thin jelly-like
layer called mesoglea that separates the epidermis from
the gastrodermis.
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