THE FLAT-FISH FAMILY 



217 



At Plymouth at a temperature of about 53° the yolk had 

 entirely disappeared, and the mouth was fully developed when 



the larva was 5-7 days old. Fie 



shows the appearance of a 



living larva in which the yolk is not yet all absorbed. 



Abundant and easily obtained as the plaice is in the later 

 stages of its life, and easy as it is to hatch the eggs, the trans- 

 formation of the larva has not yet been traced out so completely 

 and so certainly as might be desired. Such larvae have not yet 

 been reared in the aquarium, and in nature the intermediate 

 stages have not come into the hands of naturalists so frequently 

 as those of some other flat fishes. It has been mentioned that 

 at Plymouth the larv?e were kept alive for 37 days, being fed on 

 finely minced worm, and the produce of the tow-nets. But at this 



Fic. 106. — -Larva of Plaice five days old, alive, and magnified. 



age the development of the skeleton had scarcely commenced. In 

 the natural condition there is at present some uncertainty in dis- 

 tinguishing the transformation stages of the plaice from those of 

 the dab, and perhaps other species. The most easily applied 

 test in such cases, as soon as the fin-rays have appeared, is the 

 number of these fin-rays, and in the plaice and dab the number 

 is the same, while the number in the long rough dab, though 

 generally greater, varies so as to overlap the variations in the 

 other two. The plaice has not yet been found in tide pools or at 

 the margin of the sea ; until its transformation is complete, its 

 earlier stages have to be distinguished among specimens procured 

 with the tow-net either in mid-water or at the bottom. Fig. 107 

 represents the magnified appearance in the living condition of a 

 specimen taken near the Mewstone, outside Plymouth Sound, in 

 April 1 89 1. The length was f inch, and the specimen was 

 identified as a plaice by its large size. 



