Abstract: Mussels do not possess an adaptive immune system as vertebrates do.The defence systems of mussels are often described as based only in innate immunity,which includes the cellular immunity and lymph immunity.A universal classification scheme of the circulating haemocytes is a prerequisite for comparative functional studies,but by now,it has been still not available for haemocytes of mussels.In hope of knowing about the defending system of mussel and try to improve the defense capability of the mussel,the classification and the dynamic variation of the mussel haemocytes were researched.The haemocytes of H.schlegeli which was stained purple by Giemsa were observed by light microscope.At the same time,the morphology of living haemocytes was also observed.The haemocytes were classified as 6 types based on the characteristics,amount of granule and its relative size,the nucleolus/plasm(N/P) ratio.They are granulocyte,hyalinocyte,serous cell,lymphoid cell,spindly cell and thrombocyte.Abundant big or small bright granules can be observed in the granulocyte which has the most number.The hyalinocytes were showed central nucleus surrounded by relatively lucent.Filopodia of the granulocyte and hyalinocyte stretched out from any direction.The size of granulocyte((18.43±2.29)μm) was larger than that of the hyalinocyte(13.19±1.87μm),but the N/C ratio of granulocyte(0.11±0.05) was lower than hyalinocyte(0.22±0.10).The lymphoid cell was the smallest((4.45±0.69)μm) with a relatively big nucleus which is(2.85±0.77)μm,and the serous cell was the biggest((19.41±2.57)μm).The folopodia of thrombocyte stretched out usually from one side of the cell body.The sizes and N/P ratio of the spindly cell were similar with the hyalinocyte and the thrombocyte.The percentage of the 6 kinds of haemocyte was counted.The average number of the hemolymph was(1.68±0.78)×106/mL,the granulocyte(45.84±2.20)%,the hyalinocyte(36.36±3.47)%,the serous cell(5.80±1.65)%,the lymphoid cell(2.77±1.27)%,the spindly cell(5.13±2.48)%,and the thrombocyte was about(4.09±2.07)%.Results indicate that the granulocyte and hyalinocyte may take the basal metabolism and immunological function.The dynamic variation in vivo of the haemocytes was observed under the LEICA DM5000 B Microscope,namely,the morphology of haemocytes underwent manifest changes by dipping them on the slides for a short time.The hemocytes just taken out from the shell were all round-shape,and 1-2 minutes later,most of them formed pesuclopods.After about 30 minutes,the pesuclopods withdraw and haemocytes became round.In this experiment,we found the hyalinocyte and the spindly cell can transform each other.The spindly cell transform to hyalinocyte was completed about 1 hour.The degranulation from one side of cell body was a voluntary and harmless process.When the granules moved out of haemocytes,they would be dissociative and actionless.All the haemocytes began to die and disintegrate within 1h to 1.5h.