CCESSORY respiratory organs in fishes are rather seldom and usually secondarily related to living out of the water. In the rock skipping blennies, Andamnia and Erpichthys, and probably in the mud skipping gobies, Periophthalmidae, a primary specialization seems to have occurred in response to a change of environment (Hora, 1933). In all freshwater fishes with accessory respiratory structures however, the evolution of these structures has been correlated with an existence in oxygendeficient water. This has been made especially clear in several recent studies of the fishes which live in these habitats. Perhaps the most satisfactory study of this subject is that of Carter and Beadle (1931), who found that the fishes of the great Chaco swamps of Paraguay show structural modifications which enable them to breathe atmospheric oxygen when the dissolved oxygen becomes depleted. Pearse (1932) made a study of the fauna of the tidal ditches of Siam and his findings closely parallel those of Carter and Beadle. Less organized contributions show that the same kind of modifications characterize the marsh inhabiting fishes of Africa. In fact this phenomenon has been found to occur so frequently, that it may be stated as a law: fishes inhabiting fresh waters which tend to lose their dissolved oxygen possess special means for obtaining oxygen either from (1) the trace remaining in the water, (2) the aerated surface film or (3) the atmosphere. It has long been recognized (Wilder, 1878, etc.) that Amia breathes atmospheric oxygen into its air bladder. It has apparently not been pointed out that its gill filaments are highly modified, probably enabling the fish to use also the first method listed above for obtaining oxygen. Amia can presumably absorb dissolved oxygen when in very low oxygen-tension, because the gills are modified into what appears to be a most unusually effective respiratory mechanism. In Amia, each hemibranch consists of a row of longitudinally appressed filaments, and in a section of the hemibranch which is cut longitudinally in reference to the gill arch, the filaments appear honeycombed by countless perforations which measure about 0.1 by 0.05 mm. in major dimensions (Figs. 2 and 4). These oblong perforations are aligned in single files perpendicular to the gill arch. They are separated from one another by very thin membranes, which consist of a capillary2 which is invested on two sides by a single layer of low cuboidal epithelium. These thin membranes obviously represent the lamellae (leaflets) or secondary folds of the gill filaments. Normally these minute lamellae extend outward and somewhat distally to their free tips from either side of the central supporting bar of each filament, which appear in