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Ultrastructural changes during spermiogenesis in the barnacles, Ba/anus amphitrite albicostatus, Balanus tintinnabulum rosa. BaIanus trigonus and Tetraclita squamosa japonica. and organization of the sperm with special reference to the accessory body were studied. The Golgi complex organizes both the acrosome and the accessory body at different stages during spermiogenesis; the former is formed at the mid-spermatid stage and the latter is formed at the late spermatid stage. The arrangement of the components in the mature filiform sperm is quite unique, with the acrosome, the basal body just behind the acrosome, the axial filament parallel to a long nucleus, and a slender long mitochondrion behind the nucleus. The sperm in the anterior and posterior half of the ejaculatory duct differ from each other in form in that the sperm in the anterior duct are not equipped with the accessory body and the sperm in the posterior duct are. The accessory body can be artificially broken down by some treatments (1 M urea, alkaline sea water: pH 9.0-9.7, low ionic concentration of sea water). The loss of the accessory body from the sperm is assumed to be related to the fertilizability of the sperm.The structure of the crustacean sperm is remarkably varied among members of the class. BROWN (6) described the various forms of the crustacean sperm following comparative and phylogenetic studies and emphasized that the sperm of the class Crustacea can be separated into two groups, the flagellated (Cirripedia, Branchiura, Mystacocarida) and nonflagellated sperm (Cephalocarida, Branchiopoda, Malacostrace). It is widely believed that, especially in Cirripedia barnacles, the sperm is filiform with an appendage that has been called the unique vesicle (11) or the large accessory droplet (10). Recently, we observed that the sperm collected from the male ejaculatory duct of some barnacles were swimming actively by flagellar motion, and that while part of them were equipped with the appendage we named the accessory body, part lacked it.The present study was carried out in order to determine (i) whether this morphological difference in the sperm depends on the maturation of the male germ cell, (ii) how the accessory body is organized during spermiogenesis and (iii) which types of sperm are effective in fertilization.

The ultrastructure of spermatozoa and the changes through which they are differentiated during sperm formation in an echiuroid were observed under the electron microscope., Many spermatids are connected to one central cytoplasmic mass and the sperm differentiation proceeds synchronously in one sperm-ball. Dense platelike structures appear in the cytoplasm of early spermatids and disappear soon. In the process of nuclear condensation, many electron-dense aggregates appear in homogeneously textured chromonema and the aggregates are packed together to form a uniformly dense nucleus. Near the centriole at the opposite side from the central mass, the mitochondria fuse together to form one large middle-piece mitochondrion and the acrosomal vesicle is formed from the Golgi-complex. The differentiating acrosome in the late spermatid moves to the anterior tip of the head. In the completed acrosome, a flocculent substance accumulates in the conspicuously expanded invaginated pocket of the acrosomal vesicle and two kinds of material of different electron density fill the inside of the acrosomal vesicle. The spermatozoa remain connected to the central mass at the lateral side of the head until they become fully mature and are packed into the nephridia before spawning.In the phylum Annelida, the ultrastructural changes in spermiogenesis have been reported in some species of Oligochaeta by GATENBY and DALTON (1959), FERRAGUTI and LANZAVECCHIA (1971), ANDERSON et al. (1967ANDERSON et al. ( , 1969, and SHAY (1 972). In the class Echiuroidea, TYLER (1 965) reported briefly the ultrastructure of the spermatozoa of Urechis caupo and its changes in fertilization, but no electron microscope study has yet been performed on sperm formation in echiuroid worms.We intend to investigate the ultrastructural changes in gametogenesis of various marine Annelida, and in the present paper, we will report the process of the sperm formation as revealed by the electron microscope in an echiuroid species, Ikedosoma gogoshimense.As we reported previously (1962), the germ cells develop floating in the coelomic fluid in this worm. Male sexual cells make a cluster from the early stage 77