Ctenophora

Ctenophora (tĭnŏf'ərə), a small phylum of exclusively marine, invertebrate animals, commonly known as sea walnuts or comb jellies. Because they are so delicate that specimens are difficult to collect, little was known about them until the advent of blue-water scuba and submersible collecting. Ctenophores are characterized by eight rows consisting of ciliated plates called ctenes (combs), which are radially arranged on the spherical body surface. The animals swim weakly, powered by those structures. The two hemispheres of the ctenophore body are marked by a mouth, or oral pole, on the underside, and an opposite aboral pole, on which is located the statocyst, a unique sense organ controlling equilibrium. Most ctenophores resemble biradially symmetrical (see symmetry, biological) jellyfish (phylum Cnidaria) but lack the cnidarian whorl of tentacles around the mouth. They lack the specialized stinging cells (nematocysts) found in coelenterates, but one species (Haeckelia rubra) incorporates those of its jellyfish prey for its own defense. Ctenophores, which are all carnivorous, have specialized adhesive cells called colloblasts, used to capture planktonic animals on which the ctenophores feed. Approximately 50 species are known, but many become locally abundant and are ecologically significant. They vary from less than ¹/₄ in. (0.6 cm) to over 1 ft (30.5 cm) long. Most are transparent, but pale pinks, reds, violets, and oranges are also known in some species. Most ctenophores are also bioluminescent, the production of light originating in the walls of the eight canals. Most ctenostomes are hermaphrodites, developing through a cydippid larval stage to adults. They can also regenerate lost parts.

Class Tentaculata

Members of this class typically have two feathery tentacles that can be retracted into specialized sheaths. In some, there are smaller, secondary tentacles, and the primary tentacles are reduced. This class includes the small, oval sea gooseberries (genus Pleurobrachia), common on both Atlantic and Pacific coasts. The more flattened species of the genus Mnemiopsis, about 4 in. long (10 cm), is common on the upper Atlantic coast; it has a large mouth and feeds mainly on larval mollusks and copepods. This species is brilliantly luminescent. The similar, but larger, genus Leucothea is abundant on the Pacific coast. Venus's girdle (genus Cestum) is a flattened, ribbonlike form reaching over 1 yd (91 cm) in length, and found in tropical waters.

Class Nuda

This class includes species that have no tentacles. Typical is the large-mouthed genus Beröe, which feeds on jellyfish and other ctenophores.