Structure and classification of haemocytes in the bivalve molluscMeretrix meretrix

Light and electron microscopic studies were carried out in order to characterize haemocytes in the bivalve molluseMeretrix meretrix. According to nucleus and cytoplasm characters, four types of haemocytes were recognized: agranular haemocytes, lymphoid haemocyte, large granular and small granular haemocytes. Agranular hamocyte is the main cell type, accounting for 75%. It is agranular with rich organelles in cytoplasm, including mitochondria, golgi body and endoplasmic reticulum. Glycogen deposits were usually found in this cell type. The number of lymphoid haemocyte accounts for 1%–2%. This cell type is agranular and shows a high ratio of nucleus to cytoplasm. A few organelles were found. High electrondense granules with diameters of 0.2–0.5 μm and rich organelles were found in small granular haemocyte. The proportion of this cell type is about 15%. Rich granules of high electron-dense with diameters of 0.8–2.4 μm were found in large granular haemocyte. The proportion of this cell type is about 10%, and the quantity of organelles is the least.