316 A COURSE ON ZOOLOGY. 



The flat-fishes constitute an interesting family, called 

 pleuronectidae ; among all the vertebrates they are the 

 only ones that are not symmetrical. They have no 

 swimming-bladder, and, on account of the arrangement 

 of their fins, they can rest and move only on one side of 

 the body, which is flat. The two eyes are on one side, 

 and this is always the darkest, the other being white or 

 very light in color. The two sides of the mouth are 

 unequal ; swimming is accomplished by an undulating 

 movement of the whole body, and the darker side is al- 

 ways uppermost. They prefer sandy bottoms, and are 

 not found at great depths ; some live only in the sea, 

 others frequent fresh water ; they are all carnivorous. 



Fia. 273. 



HALIBUT (Hippoglossus vulgaris). 



Among the important species are the halibut, the sole, 

 not found in American waters, the turbot, the floun- 

 der, and the plaice. 



Physostomi. In this order all the fin rays are artic- 

 ulated, excepting the first ray of the dorsal and pectoral 

 fins, which is sometimes ossified. The ventral fins have 



