It is certain that fishes possess the faculty of perceiving odours, and that various scents attract or repel them. A mangled carcase or fresh blood attracts Sharks as well as the voracious Serrasal monoids of the South American rivers. There is no reason to doubt that the seat of that perception is in the olfactory sac; and it may be reasonably conjectured that its strength depends mainly on the degree of development indicated by the number and extent of the interior folds of the pituitary membrane. _Organ of Sight_.--The position, direction, and dimensions of the eyes of fishes vary greatly. In some they have an upward aspect, and are often very close together; in others they are lateral, and in a few they are even directed downwards. The Flat-fishes represent the extraordinary anomaly that both eyes are on the same side of the head, and rarely on the same level, one being generally placed more forward than the other. In certain species of marine fishes the eyes are of an extraordinary size, a peculiarity indicating that the fish either lives at a great depth, to which only a small proportion of the rays of light penetrate, or that it is of nocturnal habits. In fishes which have descended to such great depths that no rays whatever can reach them, or in freshwater fishes living in caves, or in species which grovel and live constantly in mud, the eyes are more or less aborted, sometimes quite rudimentary, and covered by the skin. In very few this organ appears to be entirely absent. In some Gobioids and Trachinoids (_Periophthalmus_, _Boleophthalmus_, _Uronoscopus_, etc.) the eyes, which are on the upper side of the head, can be elevated and depressed at the will of the fish. In the range of their vision and acuteness of sight, Fishes are very inferior to the higher classes of Vertebrates, yet at the same time it is evident that they perceive their prey or approaching danger from a considerable distance; and it would appear that the visual powers of a _Periophthalmus_, when hunting insects on mud-flats of the tropical coasts, are quite equal to that of a frog. Again, the discrimination with which fishes sometimes prefer one colour or kind of artificial fly to another affords sufficient evidence that the vision, at least of certain species is by no means devoid of clearness and precision.