Absorption of soluble food could begin in the stomach - it occurs in mammals, but has not been investigated in fish - but takes places predominantly in the midgut and probably to some degree in the hindgut. The sites and mechanisms of absorption are largely unstudied, except histologically. Several histologists have identified fat droplets in intestinal epithelial cells following a lipid-rich meal. Increased numbers of leucocytes in general circulation following a meal by the sea bream and increased number of fat droplets in them have been described (Smirnova, 1966). It was hypothesized that leucocytes entered the gut lumen, absorbed lipid droplets, and then returned to the blood stream. It is clear that the mammalian type of villi with their lymph duct (lacteal) inside are absent in fish, although there is some folding and ridging of the gut wall to increase surface area. Lacteals serve as a primary uptake route in mammals for uptake of droplets of emulsified lipids (chylomicra). Teleost fish have a lymphatic system which includes extensions into the gut wall, but its role in lipid uptake is unknown. Absorption of amino acids, peptides, and simple carbohydrates have been little studied, but presumably they diffuse through or are transported across the gut epithelium into the blood stream. What light microscopists identified as a brush border on the surface of the epithelial cells facing the gut lumen, has now been clarified with electron-microscopy as microvilli; i.e., subcellular, finger like projections of the cell membrane whose greatly increased surface area is probably involved in absorption.