Originally published in the USSR in 1986, this book contains extensive Russian references not readily available from other sources. Concentrating primarily on the less-studied free-living rather than parasitic taxa, this comprehensive study examines nematode structure and traces their morphological series from the most primitive marine forms to very specialized parasitic forms. The book also presents the classification and phylogeny of all worms that have a primary body cavity, examines the evolutionary relationships among all taxa of nematodes, and clarifies the relationships between nematodes and other groups of animals.