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(also Cyclostomi), a class of lower aquatic vertebrates of the superclass Agnatha. The eellike body is covered with a tough skin, which has many slime-secreting cells. There is no external skeleton. The notochord is persistent, and the internal skeleton is cartilaginous. There are no paired fins. The gills, which are in the form of pouches, are of endodermal origin. The funnel-shaped mouth is supported by a ring of cartilage (hence the name). The teeth are horny, and the tongue is used for perforating. The olfactory organ is sac-shaped, with a nasohypophyseal canal emerging from its bottom and ending in a blind sac. In the membranous labyrinth there are only two semicircular ducts.
The two subclasses of Cyclostomata, Petromyzones (lampreys) and Myxini (hagfishes), sharply differ from one another. Cyclostomata are marine, freshwater, or both. The females deposit eggs which are fertilized and develop in water. Cyclostomes are either ectoparasites of fishes or predators. Some species of lampreys are commercially valuable.
REFERENCESBerg, L. S. Sistema ryboobraznykh i ryb, nyne zhivushchikh i iskopaemykh, 2nd ed. Moscow-Leningrad, 1955. (Tr. Zoologicheskogo instituta AN SSSR, vol. 20.)
Nikol’skii, G. V. Chastnaia ikhtiologiia, 3rd ed. Moscow, 1971.
V. D. LEBEDEV