Abstract: Ultrastructural observation showed that a great number of long elliptical extrusomes were distributed beneath the pellicle on the dorsal and ventral sides of a hypotrichous ciliate, Oxytricha granulifera. The mature extrusomes were filled with different electron dense fibrous materials at their posterior parts, and had hollow cavities at their anterior parts. The anterior end of the mature extrusomes was close to the pellicle, and distributed in longitudinal, diagonal or arc rows. During ejection, the anterior membrane of extrusomes fused with the plasma membrane. After extrusion, the extruded contents often remained in the surface of the cells. Considered that the extruded contents of extrusomes of O. granulifera always adhere to the surfaces of the cells and its morphology has no obvious difference from the resting extrusomes, it can be supposed that during ejection, the contents may be slowly excreted toward outside through the pellicle. Therefore, the extrusomes of O. granulifera might be a kind of excretion extrusomes. Besides, the extrusomes of O. granulifera can be stained by alcian blue 8GS which can rapidly stain mucin, carboxyl and sulphate-containing mucosubstances, therefore, the extrusomes of O. granulifera may contain some mucosubstances. Under the stimulus of methyl green-pyronin, the extrusomes of O. granulifera were numerously discharged, and the extruded contents of extrusomes got together to form a "capsule" which packed the ciliate in it, thus, it might be a defensive response to harmful conditions.

Key words:  Oxytricha granulifera, extrusomes, morphology, structural components, function