CHAPTER VIII 



ADAPTATIONS 



Shape, symmetry, colour. Flat-fishes, Synodontis. Locomotion. Flying- 

 fishes. Respiration, gills and accessory organs. Air-bladder, respiratory and 

 hydrostatic function. Sense-organs and senses. Hearing, relation of auditory 

 organ to air-bladder. Sense-organs of lateral line. Sight. Divided eyes of 

 Anableps, blind fishes. 



THE most striking instance of adaptation in shape and 

 symmetry is that of the flat-fishes or Pleuronectidse. 

 These fishes habitually lie on the ground upon one 

 side of the body, some species on the left side some on the 

 right. In adaptation to this abnormal position the symmetry 

 of the two sides is in many respects altered. If a flat-fish is 

 held in a vertical position and compared with any ordinary fish 

 many of the paired organs will be seen to be present in their 

 usual position : on each side there is a lateral line, a pectoral 

 and pelvic fin, and a gill-cover or operculum. The position of 

 the gills shows which is the ventral edge of the body, and it is 

 seen that the marginal fins are continuous dorsal and ventral 

 fins, which in many species, e.g. the plaice or turbot, are distinct 

 from the the tail-fin, but in some species they are continuous 

 with the latter. The direction of the mouth also is as in other 

 fishes transverse to the median plane of symmetry, so that 

 half of the jaws is on one side, half on the other. In plaice and 

 sole, however, the two sides of the mouth are unequally de- 

 veloped, the jaws and teeth being much more strongly developed 

 on the left or lower side where they are most used for seizing 

 food. The most remarkable departure from symmetry is in 

 the eyes, both of which are on the side which is naturally 

 uppermost, that is to say the right side in a plaice or sole, the 

 left side in a turbot or brill. The eye which is nearest the 

 dorsal edge is that which belongs to the lower side, although it is 

 separated from the latter by the dorsal fin : the other eye, the 



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