THE OSMOREGULATORY TISSUE AROUND THE AFFERENT BLOOD-VESSELS OF THE COXAL GILLS IN THE ESTUARINE AMPHIPODS, GRANDIDIERELLA-JAPONICA AND MELITA-SETIFLAGELLA

By electron microscopy of the coxal gills in two species of estuarine amphipod crustaceans, Grandidierella japonica and Melita satifragella, we found a patch-like, specialized tissue area which consisted of uni que cells closely resembling the salt-excreting cells in the gill of t he brine shrimp and so-called chloride cells in teleost gills. These c ells were characterized by an abundance of mitochondria, two kinds of extensive networks of cytoplasmic tubules, well-developed lamellar inf oldings of the basal cell membrane, sparse microvillous projections of the apical border, and numerous large vacuoles with several incomplet e partitions. The large (60 nm in diameter) and the small (30 nm) cyto plasmic tubular networks were found in the basal and the apical portio ns of the cell, respectively. The large networks, which were both dire ctly and indirectly (through the lamellar system) continuous with the basal cell membrane, were regarded as extensions of the cell membrane. Both the outer walls and the partition walls of the vacuoles were rei nforced with a parallel array of microtubules. The results suggest tha t this unique tissue plays an important role in the active transport o f electrolytes to maintain a constant osmotic pressure of the hemolymp h under widely fluctuating salinities of the estuarine environments.