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a subclass or, according to another taxo-nomic system, an order of free-swimming marine coelenterates of the class Hydrozoa. The body, which ranges in length from 1 cm to 3 m, is generally colorless and transparent. Only a few species are brightly colored. Siphonophorans form polymorphic colonies whose individuals, polyps and medusae, differ in structure and are converted into organs of the colony. At the apex there is usually a swim bladder, or pneumatophore, with gas, chiefly nitrogen, which is released by the gas gland to help keep the colony vertical. In some siphonophorans, for example, the Physalia, the pneumatophore emerges at the surface of the water and functions as a sail. A colony also consists of nectocalyxes (swimming bells), which are modified medusae whose contractions propel the colony, and feeding polyps (gas-trozooids), which swallow and digest prey and bear a long tentacle (lasso cell) with a group of stinging cells. There also are sexual individuals (gonozooids). Heterogenous members of the colony are usually gathered in aggregates called cormidia. Siphonophorans reproduce sexually; the free-swimming larva, a modified planula, forms a new colony by budding.
Siphonophorans of the order Disconanta are not colonies but solitary free-swimming polyps. Of the 250 species, most inhabit tropical seas. Only two species, one from each of the genera Dimophyes and Physophora, are found in the USSR. Some large tropical siphonophorans, for example, the Portuguese man-of-war (Physalia), are dangerous to man; prolonged contact with the long (up to 10 m) tentacles in water causes severe burns and systemic disease with protracted elevation of body temperature.
REFERENCESDogel’, V. A. “Tip kishechnopolostnykh.” In Rukovodstvo po zoologii, vol. 1. Moscow-Leningrad, 1937. Pages 323-30.
Zhizn’ zhivotnvkh, vol. 1. Moscow, 1968.
Dogel’. V. A. Zoologiia bespozvonochnykh, 6th ed. Moscow, 1975.
A. V. IVANOV [23–1436–]