Ultrastructural study on cocoon formation in the freshwater oligochaete, Tubifex hattai

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Abstract
The clitellar epithelium of the freshwater oligochaete, Tubifex hattai, is composed of four types of gland cells (Type I, II, III, and IV), in addition to the cells generally found in the epidermis of this worm. The possible function of these gland cells in cocoon formation was studied with the electron microscope.Type I cells discharge their secretory granules by means of compound exocytosis and provide the materials for the future cocoon membrane. Immediately after completion of the discharge from Type I cells, Type II and III cells simultaneously discharge their secretory granules by means of compound exocytosis. The secretions from Type II cells constitute a colloid in the cocoon lumen and probably cause structural modifications in the future cocoon membrane. The secretory products from Type III cells form the cocoon plug. Although the process of discharge of secretory granules from Type IV cells was not observed, the contribution of these cells to the cocoon formation, producing hoops on the outer surface of the future cocoon membrane and fixing its anterior ends on the clitellum, is inferred from a morphological comparison of the hoop and the structure of the secretory granules.