Hemolymph cells of the bivalve mollusc Mercenaria mercenaria: an electron microscopical study.

The hemolymph cells of Mercenaria mercenaria were studied with the transmission electron microscope. Three morphological types of cells, granulocytes, hyalinocytes, and fibrocytes, are distinguishable and their fine structural characteristics are described. However, as a result of analyzing the fine structural features of the so-called fibrocytes of M. mercenaria, i.e., the inclusion of large aggregates of glycogen granules in their cytoplasm and the occurrence of primary phagosomes enclosing partially degraded exogenous material and digestive lamellae, it is suggested that fibrocytes are actually granulocytes which are at the terminal phase of their physiologic cycle relative to the degradation of phagocytized nonself materials. The cytoplasmic granules of M. mercenaria granulocytes are structurally different from those of Crassostrea virginica in that they are delimited by a unit membrane, rather than by a complex wall, and include a homogenously electron-dense material. Lipidlike droplets are reported from both granulocytes and hyalinocytes of M. mercenaria for the first time.