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The larva of the endemic Baikal sponge Swartschewskia papyracea freely swimming in water is a parenchymula which contains all main types of cells peculiar for the definitive sponge. Ultrastructure of these cells is described. Flagellar cells of the larva form a continuous layer with signs of polarity in distribution of organoids and inclusions specific for epithelial cells. Early specialization of collencytes secures, already at the larval stage, synthesis of a specific protein--collagen. Cytoplasm of blastomere-like nucleolar amoebocytes is overloaded with yolky inclusions at various stages of digestion. A developed Golgi complex is characteristic for this type of cells. Amoeboid cells with peculiar inclusions limited with a membrane and filled with a fine fibrillar material having central electron opaque granules are revealed.

In the sponge Lubomirskia baicalensis, a representative of the endemic fauna in the Baikal Lake, male specimens were first discovered and successive stages of their spermatogenesis were followed at light and electron microscopic levels. Male sex cells develop in cysts surrounded with a membrane consisting of cells which resemble pinacocytes by their ultrastructure. One sponge, during the period a spermatogenesis, contains in its tissue testicular follicles at various stages of the male sex cells maturation, within every follicle the development occurring more or less synchronously. As a result of transformations taking place in the ultrastructural organization of the spermatid, a primitive type of spermatozoon is forming, without acrosome, with large amount of cytoplasm containing small mitochondria in it.