ИСТИНА

Oral presentation. ABTRACT Microscopic anatomy and ultrastructure of body cavity and adjacent organs in a sea spider Nymphon brevirostre Hodge, 1863 were studied by TEM. Longitudinal septa subdividing body cavity are described: 1) horizontal “Dohrn’s” septum; 2) lateral heart walls; 3) paired ventral septa formed of separate cellular bands. Body cavity appears to be hemocoel which has no epithelial lining but only bordering basal lamina which underlays epidermis, covers Dohrn’s septum, nervous system, gut and its adjacent musculature, both inner and outer sides of heart walls and lines hemocoel lacunae in Dohrn’s septum. Epidermis, heart and Dohrn’s septum are not separated from each other by a basal lamina and form continuous tissue system of probable epidermal origin. Cellular bands forming longitudinal ventral septa are not covered with basal lamina and presumably originate from amebocytes that let us consider them as structures of mesodermal origin. All longitudinal septa are supposed to promote proper hemolymph circulation. The gonads are incorporated into the Dohrn’s septum. Hemocoel lacunae enter into the Dohrn’s septum and surround gonads. Basal lamina covers the gonad from the outside. The gonad wall is formed of a single-layer epithelium. Muscle cells and a rudimentary cilium were found in gonad wall epithelium. The gonocoel is similar to a coelomic cavity because it is not lined with a basal lamina. Free-circulating cells of two types are found in hemocoel: 1) small amebocytes containing electronic-dense granules are supposed to be granulocytes similar to those of other arthropods; 2) hemocytes with large vacuoles of varying structure may be compared with plasmatocytes but some of them may turn out to be activated granulocytes