48 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



The " why " of the peculiar metamorphosis of the Pleuronectidso is an 

 unsolved problem. The presence or absence of a swim bladder can have 

 nothing to do with the change of habit of the young flatfish, for P. 

 americanus must lose its air-bladder before metamorphosis begins, since 

 sections showed no evidence of it, whereas in Bothus the air-sac can 

 often be seen by the naked eye up to the time when the fish assumes 

 the adult coloration, and long after it has assumed the adult form. 



Cunningham ('92-97) has suggested that the weight of the fish acting 

 upon the lower eye after the turning would press it towards the upper 

 side out of the way. But in all probability the planktonic larva rests 

 on the sea bottom little if at all before metamorphosing. Those taken 

 by me into the laboratory showed in resting no preference for either side 

 until the eye was near the mid-line. 



That the change in all species is repeated during the development of 

 each individual fish, has been used to support the proposition that the 

 flatfishes as a family are a comparatively recent product. They are, on 

 the other hand, comparatively ancient. According to Zittel ('87-90, pp. 

 315-316) flatfishes of species referable to genera living at present, 

 Rhombus and Solea, are found in the Eocene deposits. These two 

 genera are notable in that Rhombus is the least and Solea the most 

 unsymmetrical of the Pleuronectida). 



The degree of asymmetry can be correlated with the habit of the ani- 

 mal. Those fishes, such as the sole and the shore-dwelling flounders, 

 wliich keep to the bottom, are the most twisted representatives of the 

 family, while the more freely swimming forms, like the sand-dab, summer 

 flounder and halibut, are more nearly symmetrical. Asymmetry must 

 be of more advantage to those fishes which grub in the mud for their 

 food than to those which capture other fishes ; of the latter, those that 

 move with the greatest freedom are the most symmetrical. 



This deviation from the bilateral condition must have come about 

 either as a "sport," or by gradual modification of the adults. If liy the 

 latter method, — the change proving to be advantageous, — selection 

 favored its appearing earlier and earlier in ontogeny, until it occurred in 

 the stages of planktonic life. Metamorphosis at an age younger than this 

 would be a distinct disadvantage, because of the lack of the customary 

 planktonic food at the sea-bottom. At present some forms of selection 

 are probably continually at work fixing tlie limit of the period of meta- 

 morphosis by the removal of those individuals which attempt the trans- 

 formation at unsuitable epochs, — for instance, at the time of hatching. 

 That there are such individuals is shown by FuUarton ('91), who figures 



