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, the larvae of which develop into medusalike organisms from which the colony arises. The individual members of a siphonophoran colony are extremely specialized; the polyps specializing in defense produce a venom strong enough to kill a human being. Another order has a short-lived polyp stage; in some species no polyp stage has yet been observed. Its members spend most of their lives as free-swimming medusae. A fourth order combines traits of polyps and medusae, somewhat resembling free-swimming, ciliated hydras. The two primarily tropical orders contain a number of interesting forms that secrete a large, hard, calcareous exoskeleton and often form corals.

Scientific classification: The class Hydrozoa belongs to the phylum Cnidaria. Hydra belong to the order Hydroida, siphonophores to the order Siphonophora. Species with a short-lived or nonexistent polyp stage belong to the order Trachylina. Species resembling free-swimming polyps belong to the order Actinulida. The two primarily tropical orders are Stylasterina and Milleporina; the latter is sometimes considered a suborder.

Hydrozoa

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, the larvae of which develop into medusalike organisms from which the colony arises. The individual members of a siphonophoran colony are extremely specialized; the polyps specializing in defense produce a venom strong enough to kill a human being. Another order has a short-lived polyp stage; in some species no polyp stage has yet been observed. Its members spend most of their lives as free-swimming medusae. A fourth order combines traits of polyps and medusae, somewhat resembling free-swimming, ciliated hydras. The two primarily tropical orders contain a number of interesting forms that secrete a large, hard, calcareous exoskeleton and often form corals.

Scientific classification: The class Hydrozoa belongs to the phylum Cnidaria. Hydra belong to the order Hydroida, siphonophores to the order Siphonophora. Species with a short-lived or nonexistent polyp stage belong to the order Trachylina. Species resembling free-swimming polyps belong to the order Actinulida. The two primarily tropical orders are Stylasterina and Milleporina; the latter is sometimes considered a suborder.

Types of Cnidarians

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, inconspicuous polyps. Polyps reproduce asexually to form a medusa—saucer-like structures bud off the polyp and swim away as new medusae.

Cubozoa
The medusa phase dominates in animals in the Cubozoan class. Cubozoan medusae are commonly called box jellies because they have a cube shape with a single tentacle or group of tentacles hanging from each corner at the mouth end of the animal. The Cubozoan polyp is small and inconspicuous and an entire polyp transforms into a medusa in a type of asexual reproduction. Cubozoa consists of about 20 species of sea wasps and their relatives. The nematocyst sting of some cubozoans, including the Australian sea wasp Chironex fleckeri, contains poison that can be fatal to humans.

Anthozoa
In the Anthozoa class there is no medusa stage. An anthozoan polyp reproduces sexually, although the polyps of some species also reproduce asexually. Asexual reproduction results in a colony if the daughter polyps remain attached to one another. There are about 6,500 species of anthozoans, including corals, sea anemones, sea pens, and sea fans (see Gorgonian).