The polyp's mouth and tentacles are located anteriorly, facing upwards. The first is the polyp stage in this phase, the jellyfish takes the form of either a sessile stalk that catches passing food, or a similar free-floating configuration. Most jellyfish pass through two different body forms during their life cycle. The name "jellyfish" is also often used to denote two other classes of cnidarians, Class Hydrozoa (Portuguese Man o' War, Obelia, etc.), or Class Cubozoa (box jellyfish or box jellies). Since jellyfish do not biologically qualify as actual " fish", the term "jellyfish" is considered a misnomer by some, who instead employ the names " jellies" or " sea jellies". The size of the cups or bells can range from two centimeters across to four meters, and some species trail tentacles for many meters below in the water (Towle 1989). Jellyfish can be found in every ocean in the world. Some, such as Rhizostomae, do not have tentacles at all. Sea wasp jellyfish have killed dozens of people along the coast of Australia (Towle 1989). Most jellyfish use them to secure prey or as a defense mechanism. Each tentacle of a jellyfish is covered with these stinging cells, which can sting or kill other animals. The name of the phylum comes from cnidocytes, or nematocysts, which are specialized cells that carry stinging organelles. There is some concern that blooms of jellyfish-congregating of hundreds and even thousands of these animals-correlates with such anthropogenic impacts as overfishing and pollution. The term comes from the Greek word skyphos, denoting a kind of drinking cup. The class name Scyphozoa means "cup animals," reflective of the dominant medusa form (Towle 1989). However, the Scyphozoa live most of their life cycle as medusa. Theoretically, members of Cnidaria have life cycles that alternate between asexual polyps (the body as a vase shaped form), and sexual, free-swimming forms called medusae (singular medusa the body in a bell-shaped form). The body of an adult jellyfish is composed of a bell-shaped, jellylike substance enclosing its internal structure, from which the creature's tentacles suspend. Jellyfish are marine invertebrates belonging to the Scyphozoan class of the Cnidaria phylum.
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