ULTRASTRUCTURAL-STUDY OF 2 GLANDULAR SYSTEMS IN THE PROBOSCIDIAL GLANDULAR EPITHELIUM OF RISERIELLUS-OCCULTUS (NEMERTEA, HETERONEMERTEA)

Two different types of glandular system in the proboscidial epithelium of Riseriellus occultus have been investigated by transmission electr on microscopy. As expected, most of the epithelial cells are glandular in nature. With regard to differences in the ultrastructure of these gland cells and in the formation and morphology of their secretory gra nules, we have categorized and described four types of gland cell, ind icated as G(1), G(2), G(3), and G(4). Each gland cell has a completely intraepithelial body characterized by a prominent nucleus, developed rough endoplasmic reticulum, Golgi complexes, and numerous secretory g ranules at different stages of maturation. These four types of gland c ell appear associated in pairs forming numerous glandular systems of t wo types (A, B). These glandular systems are restricted to the ventral surface of the proboscis and are scattered irregularly throughout its length. Each glandular system consists of two gland cells of differen t types. The gland cell necks in each glandular system extend together to the epithelial surface; they protrude onto this and form a papilla where they open in a common area. The epithelial supportive cells adj acent to the glandular systems have long, stout microvilli which have a core of tonofilaments. These tonofilaments gather into dense bundles which pass vertically through the supportive cells and attach to the ex tracellular matrix underlaying the cells by hemidesmosomes. Moreove r, a single sensory process stands close to each papilla. The ultrastr uctural morphology of the type A glandular systems suggests that they have an adhesive function operating in a similar way to that of the du e-gland adhesive systems in other invertebrate groups, although they a re not homologous with these. The spatial arrangement of the secreted products of the type B glandular systems suggests that these may contr ibute to increasing the grip of the proboscis on the prey. The secreto ry granules (=pseudocnids) of the type G(3) gland cells are very likel y an autapomorphy of the Anopla, providing a character by which the re lationships within the Nemertea can be evaluated.