Structure of the Hermaphroditic Gonads in a Japanese Cephalocarid Crustacean, Sandersiella sp. with Special Reference to the Oogenetic Mode

Abstract

Structure of the adult gonads of a hermaphroditic cephalocarid, Sandersiella sp., was examined and some phylogenetic features on the mode of oogenesis were discussed. Both a pair of ovaries and a pair of testes lay along the alimentary canal. A long sac-like ovary was located in each side of the alimentary canal, extended from the eighth to the 18th and the 19th postcephalic segment. A short oviduct grew forward from the anterior end of each ovary. A pair of short sac-like testes was situated on the dorsal side of the alimentary canal, extended from the seventh to the eleventh postcephalic segment. A short spermiduct arose forward from the anterior end of each testis and then was joined with the oviduct to form a common genital duct opened at the base of the sixth thoracopod. The ovarian wall was a thin single-layered ovarian epithelium. Three and four germaria including several oogonia and very early previtellogenic oocytes embedded in the ovarian epithelium, we rearranged in a line along the ventromedian axis of each ovary. Approximately 10 larger oocytes at various oogenetic stages, previtellogenic and vitellogenic, lay in the ovarian lumen other than the younger oocytes in the germaria. Neither follicle cells nor nurse cells were found about these oocytes. A mature egg oviposited was attached to each gonopod with cement. In the present specimen, the early previtellogenic oocytes should leave the germaria to grow into mature eggs in the ovarian lumen, which a mature egg should be oviposited through each of the oviducts, the common genital ducts, and gonopores. Such an oogenetic egg laying mode is of the typical mandibulate-type, which has been expected in the cephalocarids, one of the most primitive mandibulate arthropods.