ADAPTATIONS 373 



torsion of the skull in the region of the eyes through an angle 

 of 90°, and this twisting has taken place without affecting the 

 symmetry of the posterior region of the skull, or the jaws and 

 anterior region. The interorbital septum of the skull has been 

 bent round from the middle line to the right side, in the sole 

 for example, and the eyes lie on either side of the septum as 

 usual. The bones which were originally below and external to 

 the left eye have become larger, and a new connection on this 

 side has been formed between the bones of the anterior and 

 posterior region of the skull. Along this bony ridge which 

 is now in line with the original dorsal edge of the body, the 

 dorsal fin has extended, thus dividing the originally left eye 

 from the left side of the face. The jaw-bones, instead of being 

 twisted in the same manner, have actually become much larger 

 on the lower side, so that the asymmetry of the mouth is in the 

 opposite direction to that of the eyes. 



The use of the tail as chief organ of locomotion is not suit- 

 able for a fish gliding along the ground, and we find both in 

 skates and flat-fishes that the fish moves by undulations of 

 lateral fins at the margins of the flattened body. The tail or 

 caudal fin in both cases has become reduced, and the lateral 

 fins much increased in extent. In the skates it is the pectoral 

 fins which are lateral and are thus increased, in the flat-fish 

 lying on its side the marginal fins are the dorsal and ventral, 

 which have become increased so much that they extend from 

 the base of the tail nearly to the anterior extremity. The dorsal 

 fin in the sole extends to the anterior extremity of the head, 

 in the turbot nearly as far, and even in the plaice and its allies 

 as far as the middle of the dorsal eye. The ventral fin cannot 

 extend so far forward, as it cannot pass in front of the anus, 

 but the latter opening itself is shifted forward as far as possible 

 so as to be near the gill-openings, and the pelvic fins occupy 

 more or less of the small extent of margin left between the 

 anus and the gill-apparatus. The dorsal and ventral regions of 

 the flat-fish are thus much more similar than in other fishes, 

 and in fact a new though imperfect symmetry is produced about 

 the middle line of the upper side, which has to some extent 

 replaced the original symmetry between the two sides ; the 

 bilateral symmetry characteristic of a vertical fish has given 

 place to a dorso-ventral symmetry in the horizontal fish. 



