Abyssal Sipunculids (Genus Phascolion Theel) of the North-Western Part of the Pacific Collected by "Vitjaz" Expeditions in 1950-1955

As the result of abyssal trawling two Sipunculid-species 'of the genus Phascolion Phascolion lutense Selenka, 1885, and a new species designated Phascolion pacificum — were got by the expeditions of the Institute of Oceanology, Academy of Sciences of the USSR. Phascolion lutense has been previously found by the «Challenger» expedition in 1874 in the southern part of the Indian Ocean at the depth of 3.568 and 3.594 m. The finding of this species in a mass quantity in the Bering Sea, in Aleutian, Kuril-Kamchatka and Japanese trenches and on the oceanic plateau in the north-western part of the Pacific at the depth of 3.606—7.340 m significantly expands its horizontal and vertical ranges. Quantitative method of the analysis of Phascolion lutense populations from diffe­rent Pacific trenches shows that all its exemplars, in spite of their great variability, belong to one species. The new species Phascolion pacificum is found only in the Kuril-Kamchatka and Japanese trenches at the depth of 5.080—7.340 m. On the basis of counting the hooks of the introvert in the exemplars of different di­mensions belonging to both species the regularity previously noted in literature is proved, — falling out of the hooks of the introvert in larger, i. e. older exemplars. Thus the opinion of certain authors (Theel, 1911; Southern, 1913) that abyssal species are an exception to this rule, is being rejected