Electron microscopic study of the open circulatory system of the shrimp,Caridina japonica. I. Gill capillaries

The ultrastructure of the phyllobranchiate type gill of the shrimp, Caridina japonica, was studied. The most characteristic feature of the open circulatory system of Cardina is the vascular lumen of the gill capillaries which is considered to be the interstitial space. The following observations substantiate this view: (1) a thin fibrous layer forms the innermost structure of the walls of gill capillaries and is in direct contact with the blood stream; (2) filaments in the fibrous layer are assumed to correspond to the reticular fibers in the interstitial space of the alveolar wall of mammals; (3) the absence of the endothelium as well as the endothelial basal lamina which are the essential structural components of the closed circulatory system in vertebrates. The gill epithelium contains intermediate, septate and tight junctions. The first two form a junctional complex near the apical cell border and may function as a permeability barrier by occluding the intercellular space as well as functioning in electrical coupling and cellular adhesion. The tight junction is spot-like and may serve no role in the function of the permeability barrier.