LARV^ AND METAMORPHOSES 21 



had adopted the habit of fixing themselves to the rocks of the coast, 

 and which, in the course of their development, show memories of 

 their high descent. But it is also possible to suppose that their 

 history has been different. It would be greatly to the advantage of 

 animals which are anchored in adult life if their young could move 

 about and settle down in new, less crowded, and perhaps more 

 suitable quarters. The swimming shape is no peculiarity of verte- 

 brates, and this tail and the directing sense-organs may be new 

 characters acquired for the purposes of the larva. 



Flat-fish like the sole and the turbot show a metamorphosis 

 which is more easy to understand, and which occurs when the kind 

 of life led by the larvas changes to that of the adults. Most bony 

 fishes have what we think of as the usual shape of a fish. They are 

 symmetrical, with the right and left sides of the body alike in shape, 

 colouring, arrangement of the fins and such paired organs as the 

 eyes. Whether they live near the surface of the sea, or haunt the 

 bottom, they swim in the same sort of position as we do when we are 

 using the ordinary breast-stroke, that is to say, the back is upwards, 

 the under side is downwards. The upper side, too, is much more 

 darkly coloured than the white or very pale under side. The newly 

 hatched larva^ of turbot, briU, halibut, plaice, soles and other 

 flat-fish have this familiar and symmetrical shape and coloration, 

 and when they begin to feed, pursue their small prey in the water 

 exactly like other predaceous fishes. When they have grown to a 

 little less than half an inch in size, however, a sudden change comes 

 about. The right and left sides of the body become very different. 

 In the turbot and briU the left side, and in the halibut, plaice and 

 sole (Fig. 10) the right side, become dark in colour, whilst the other 

 side loses any pigment it had and is almost completely white. The eye 

 of the uncoloured side rapidly moves, partly round and partly through 

 the head, until it comes to lie near the other eye on the coloured side 

 of the body. At the same time other changes in the shape of the 

 body and the position of the organs take place, so that the sym- 

 metrical larva becomes a distorted adult, what we would call at 

 first sight the upper side not being the real back of the animal, but 

 the right side in some cases, the left side in others. When the 

 metamorphosis is complete, the fish changes its habits. Instead 

 of swimming freely through the water, it lurks on the bottom, lying 

 flat on the sand or mud, with the coloured side uppermost. In 

 these cases there can be almost no doubt but that the larva, which 

 is like the great majority of fish, is the ancestral form, and that the 



