FURTHER OBSERVATION OF A RECENTLY FOUND SENSE ORGAN IN SOME EUNICIFORMS, WITH SPECIAL REFERENCE TO LUMBRINERIS-LONGIFOLIA (POLYCHAETA, LUMBRINERIDAE)

Extensive ultrastructural observations of a presumed sense organ, rece ntly described at the ventral base of each dorsal cirrus in some eunic iforms, were made in this study, especially focusing on the organ in L umbrineris longifolia. Different from most euniciforms, this species e ntirely lacks dorsal cirri and the organ is situated directly on the d orsum at the base of each parapodium. The ultrastructure of this organ is also quite different from that of Marphysa sanguinea, described in our previous paper, in several ways. Tn Lumbrineris longifolia this o rgan is poorly developed externally but the basal part is very conspic uous extending deeply from the surface. The organ consists mainly of s everal sensory and supporting cells. About 70 sensory processes were c ounted in a single organ and each of them is equipped with a collar of 10 microvilli. Of the sensory cells found in this organ in L. longifo lia, two types were discriminated; one is ''the central sensory cell'' and the other is ''the peripheral sensory cell''. While the basal pro cesses of the former type cells have not been detected yet, the latter type cells definitely have those, indicating that they rue the primar y sensory cells. In addition one of the latter cells clearly showed to have two apical processes. The unique feature of the former type cell s is that the perikarya penetrate deeply into the subdermal tissues wi th mucous cells surrounding them like a bag. The cell membrane of the mucous cell has fenestrae formed by fusion with the border membrane of mucous globules. Mucus may be secreted through these fenestrae into t he interstitial space continuous with subdermal blood vessel. In spite of lacking detection of the basal processes of the central sensory ce lls in this study, it seems logical to consider that the mucous cells surrounding the perikarya of the central sensory cells of the organ ca n be a kind of effector cells.