ADAPTATIONS 397 



the branchial sense-organs were originally adapted to the per- 

 ceptions of vibrations of the water, but we may regard the ear 

 and the lateral line organs as exhibiting different degrees of 

 modification of the same kind of sense-organ, and in their present 

 state they are sensitive to vibrations of different frequencies. 

 The auditory organ is thus a lateral line organ which has become 

 more highly developed and adapted for the perception of the 

 delicate vibrations of sound. 



One of the most extraordinary adaptations of the eyes of 

 fishes is that of Anableps tetrophthalmus, which are divided into 

 two parts, the upper adapted for vision in the air the lower for 

 vision in water. The eyes are large, and projecting across the 

 surface of the transparent front part called the cornea is a 

 horizontal pigmented band, the pupil is divided by a correspond- 

 ing partition formed by anterior and posterior projections of the 

 iris, and it is said that the curvature of the lower half of the 

 lens is more convex than that of the upper part. This horizontal 

 division of the eyes is exactly in line with the dorsal surface of the 

 fish which is perfectly straight, and is exactly level with the sur- 

 face of the water as the fish swims. (Plate XXXI., B.) This fish 

 inhabits the rivers and estuaries of Central and South America, 

 and its habits are thus described by an observer: They swim 

 always at the surface of the water and in little schools arranged 

 in platoons or abreast. They always swim headed upstream 

 against the current, and feed upon floating matter which the 

 current brings them. A platoon may be seen in regular forma- 

 tion breasting the current, either making slight headway up- 

 stream or merely maintaining their station, and on the qui vive 

 for any food the current may bring. Now and then one may 

 be seen to dart forward, seize a floating particle and then resume 

 its place in the platoon, and thus they may be observed feeding 

 for long periods. They are almost invariably found in running 

 water well out in the stream, or at least where the current is 

 strongest and where floating matter is most abundant, for it is 

 upon floating matter that they seem chiefly to depend. They 

 are not known to jump out of the water to catch insects flying in 

 the air or resting upon vegetation above the water surface, nor 

 do they seem to feed to any extent upon small crustaceans or 

 other portions of the plankton beneath the surface. Rarely do 

 they attempt to dive or to get beneath the surface ; when they 



