FRESHWATER JELLYFISH

any medusa, or free-swimming form, of the genus Craspedacusta, class Hydrozoa (phylum Cnidaria). Craspedacusta is not a true jellyfish; true jellyfish are exclusively marine in habit. Craspedacusta sowerbyi, which is widespread in freshwaters of the Northern Hemisphere, grows to about 2 centimetres (0.8 inch) in diameter. Several hundred short tentacles extend, fringelike, from the margins of the animal's bell-shaped body. The medusoid form alternates in generations with a polypoid form (cylindrical, stalklike, and permanently attached to a surface). The polyp, about 2 millimetres (0.08 inch) high, produces a medusoid form by budding; i.e., an extension of the main body breaks away. The medusa is either male or female, rather than bisexual, and produces eggs or sperm. These unite to form a planula, a free-swimming larva, that settles onto a surface and develops into a polyp. The Craspedacusta polyp is notable for its lack of tentacles.