Kids Research Express

Hydrozoa, class of invertebrates belonging to the cnidarian phylum. The hydrozoans make up six orders. Most species of hydrozoans show alternation of generations . One generation consists of individuals called polyps and is almost always colonial; the colony is known as a hydroid because of the resemblance of its individual polyps to the noncolonial hydra , the simplest of all hydrozoans. The other generation, members of which are known as medusae, consists of free-swimming, sexual individuals that produce either eggs or sperm; the familiar jellyfish is a typical medusa. Each fertilized egg develops into a ciliated larva (planula), from which the hydroid develops and then gives rise to medusae by budding. Hydrozoans differ in their conformity to the typical alternation of generations. The order to which the hydra belongs has small medusae and sometimes no free-swimming forms at all. The siphonophoran order, containing the Portuguese man-of-war , has a short-lived medusa stage, the la...

Hydrozoa Hydrozoa, class of invertebrates belonging to the cnidarian phylum. The hydrozoans make up six orders. Most species of hydrozoans show alternation of generations . One generation consists of individuals called polyps and is almost always colonial; the colony is known as a hydroid because of the resemblance of its individual polyps to the noncolonial hydra, the simplest of all hydrozoans. The other generation, members of which are known as medusae, consists of free-swimming, sexual individuals that produce either eggs or sperm; the familiar jellyfish is a typical medusa. Each fertilized egg develops into a ciliated larva (planula), from which the hydroid develops and then gives rise to medusae by budding. Hydrozoans differ in their conformity to the typical alternation of generations. The order to which the hydra belongs has small medusae and sometimes no free-swimming forms at all. The siphonophoran order, containing the Portuguese man-of-war, has a short-lived medusa stage, t...

Scientists divide cnidarians into four classes: Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa. They base this division partly on whether the polyp or medusa is more conspicuous during an animal’s life cycle. Hydrozoa In Hydrozoa, the polyp phase dominates in the animal’s life cycle. Most hydrozoan polyps reproduce by budding to form a polyp colony that fastens to solid objects such as rocks and pilings. Some hydrozoan polyps also bud to form a medusa, which is generally small and lives only a short time. This class comprises about 3,100 species, including the freshwater hydra (which lacks a medusa stage), the Portuguese man-of-war, and fire coral. Scyphozoa The medusa phase dominates in most of the 200 species of scyphozoans, the cnidarians most frequently referred to as jellyfish. Scyphozoan medusae are larger and have a more complicated structure than the short-lived medusae of the hydrozoans. Scyphozoan medusae live several months or more, reproducing sexually to form small, inconspicu...