Jellyfish, any planktonic marine member of the class Scyphozoa (phylum Cnidaria), a group of invertebrate animals composed of about 200 described species, or of the class Cubozoa (approximately 20 species). The term is also frequently applied to certain other cnidarians (such as members of the class Hydrozoa) that have a medusoid (bell- or saucer-shaped) body form, as, for example, the hydromedusae and the siwphonophores (including the Portuguese man-of-war). Unrelated forms such as comb jellies (phylum Ctenophora) and salps (phylum Chordata) are also referred to as jellyfish. Scyphozoan jellyfish can be divided into two types, those that are free-swimming medusae and those that are sessile (i.e., stem animals that are attached to seaweed and other objects by a stalk). The sessile polyplike forms constitute the order Stauromedusae.