FISHES AND SEA-SQUIRTS. 313 



coloured surface and one on the lower white surface; the 

 pelvic fins in an ordinary fish lie on the ventral (under) 

 surface of the body, therefore in the flounder we find them 

 both close together on that sharp edge which structurally, 

 though not actually, is the under surface of the fish. So 

 far all is relatively simple, but one naturally asks, What of 

 the eyes'? It is obvious that if they were to occupy the 

 normal position we should get one on the upper pigmented 

 surface, and one on the lower colourless surface, where, 

 owing to the ground habitat, it would be useless. In point 

 of fact, both eyes occur on the surface which is normally 

 uppermost, but this is accomplished by one of the most 

 remarkable phenomena in the development of fishes, the 

 gradual migration of the originally lower eye to the pig- 

 mented surface. The migration occurs during the early life 

 of the flounder, when the bones of the head are soft, and 

 results in an extraordinary distortion of the skull. Skulls 

 of some of the flat-fish may often be found on the shore, 

 and should be studied with special reference to the position 

 of the orbits. Similarly, while the young flounder has 

 pigment on both surfaces, later the under surface (left side) 

 becomes colourless, and the pigment is concentrated on the 

 upper surface (right side). 



In some ways one of the most interesting points about 

 the flat-fish is the approach they make to a new type of 

 symmetry. It is obvious that fish, like so many animals, 

 are bilaterally symmetrical that is, the two sides are similar 

 to each other mirror images of one another. But in flat- 

 fish this similarity is no longer obvious, and the animals 

 tend to take on a type of symmetry in which the ventral 

 and dorsal surfaces resemble one another. Thus while in 

 most fish the ventral fin differs in appearance from the 

 dorsal, in the flat-fish it tends to be closely similar. Space 

 does not, however, permit of a detailed account of the 

 peculiarities of the flat-fish, or a discussion of the many 

 interesting points connected with them, and the disputes 

 to which they have given rise. 



Small flounders are common in sandy pools, especially 

 about the mouths of rivers. They may be distinguished 

 from young plaice by the fact that the scales are rudimentary, 

 and that there is a row of tubercles at the bases of the 



