The blood capillary forms the axis of the secondary lamella. It is kept open by a particular kind of cells, i.e. the so-called Bietrix pillar cells, between which the blood circulates. These cells are column-shaped and support the two sides of the secondary lamella. They also have laminar extends at each end. The cylindrical body of the pillars contains the nucleus, and reaches the two basal laminae. The pillar cells have a large nucleus, the cytoplasm is generally scanty, and the organula are never well distinguished. These cells are evenly distributed, so that, in section cut parallel to the lamella development (fig. 3), one can see the blood flowing through a round meshed lattice composed of pillar cells. Therefore the blood flows through cavities bordered merely by these cells, which are endothelial. The pillar cells follows directly the endothelium of the afferent and efferent lamella vessels. Between the epithelium and the vascular axis there is a basal membrane, which delineates the whole lenght of the blood capillary, penetrating within the gill filament for a tract corresponding to the thickness of the filament body epithelium. The basal membrane shows some tubular indentations, which penetrate within the pillar cells cytoplasm. Nevertheless these extends are not endocellular, since they are clearly delimitated by the plasma membrane.