1. The free border of the intestinal absorptive cells of cyprinids is nearly similar to that in tetrapods. This border has been described for four other species of fish and the possible evolution of the border from a ciliated epithelium has been described.
2. The mitochondria of the absorptive cells are arranged in a supranuclear group of mainly rod-like bodies and a subnuclear group of mainly granular ones.
3. The mitochondria are coarser and the Golgi element more readily impregnated with osmium tetroxide in the carnivorous Gobio than in the omnivorous Rutilus or the herbivorous Cyprinus. In all three fish both organoids are associated with fat absorption.
4. The goblet cells secrete both mucus and zymogen. Their stomata are permanently open.
5. Granular cells are described in Rutilus and Cyprinus, and in seven other species of fish, but they are absent from Gobio. Their varying reactions to numerous cytological techniques are described.
6. Alkaline phosphatase is especially concentrated in the free border of the absorptive cells. It occurs along the whole length of the gut. Its possible role in the absorption of fat and glucose is discussed.
7. Lipase is secreted by the absorptive cells. Fat absorption occurs along the whole length of the intestine, but the second limb of the intestine is especially active in this respect.
8. Carbohydrases and lipases are more concentrated towards the anterior end of the gut, whereas proteinases are more abundant caudally.
9. Carbohydrases are richer in Cyprinus and Rutilus than in Gobio.
10. Zymogen and lipase secretions are more concentrated in Gobio (carnivorous) than in the other two species.
11. The pH of the intestine of all three fishes is round about neutral point. Free HCl is absent from all three.