1. Contrary to some reports, a rectal gland is present in strictly freshwater stingrays of South American rivers (Potamotrygon spp.). 2. The gland has the location and histological features of the salt-secreting rectal gland of marine elasmobranchs, but is much reduced in size and number of tubules. 3. Its residual function, if any, is unknown. 4. The rectal gland is associated with prominent myeloid lobes, here designated as rectomyeloid bodies. 5. In the absence of potamotrygonid fossils, the atrophied rectal gland is strong evidence of marine ancestry for the freshwater rays. 6. Both the reduced gland and the loss of urea retention in potamotrygonids are indicative of a long history of freshwater adaptation.
Kidney tissue from the freshwater chondrostean fish, Polyodon spathula, was examined by light microscopy. P. spathula has a glomerular excretory kidney. The tubule leading from the glomerulus, based on a variety of stain reactions, is composed of two major cell types. Also based on stain reactions, it appears that ion reabsorption, hence conservation, occurs in distal portions of the tubules. Also observed were stain reactions of arteriole walls, similar to renin producing cells in other vertebrates.
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