OVARIAN STRUCTURE AND OOGENESIS IN THE MYODOCOPID OSTRACOD VARGULA HILGENDORFII

Abstract

A pair of laterally flattened saclike ovaries separately lie on both sides of the alimentary canal in the posterior region of the trunk of the myodocopid ostracod Vargula hilgendorfii. Neither the oogonia nor the germarium as a young germ-cell cluster are found in the adult ovary. Several tens of synchronously growing oocytes, previtellogenic or vitellogenic, are found in the ovary, not in the ovarian lumen but on the outer surface of the ovarian wall facing the alimentary canal. Each of these oocytes is protruded outward from the ovarian wall, supported by a short stalk, a short protrusion of the ovarian epithelium composing the ovarian wall. These stalked oocytes are tightly covered by the extremely extended basement membrane of the ovarian epithelium. In the ovary of the fifth instar juveniles of V hilgendorfii, the oogonia and very early previtellogenic oocytes are scattered among the ovarian epithelial cells of the ovarian wall facing the alimentary canal. Several tens of larger stalked previtellogenic oocytes of mostly similar size are protruded toward the alimentary canal. The oogenetic mode of V hilgendorfii, in particular the growth of the oocytes on the outer surface of the ovarian wall, seems similar to those in branchiurans, pentastomids, and chelicerate arthropods, rather than to those in many other crustaceans. However, the ovary of V hilgendorfii has a basic paired saclike structure, common to that in many other crustaceans, but different from unpaired tubular ovaries in chelicerates.