1054 FRESH-WATER BIOLOGY 



which quiet pools alternate with swift-flowing ripples. In the 

 Mississippi Valley the riffles are occupied by darters, in Cuba by 

 gobies, in South America by characins, and, although belonging 

 to widely different families, they greatly resemble each other. 



Light and Adjustments to It. In the shallower parts of clear 

 water the fluctuations in light from day to night are but little less 

 than in the air. Various fishes are variously adjusted to the 



FlG. 1545. Hog Sucker, Catostomus nigricans Le Sueur. Actual size, 305 mm. long. 



light. Some are nocturnal, remaining hidden during the day, as 

 the common catfish. Some combine stereotropism with their neg- 

 ative heliotropism, and take shelter in crevices and under rocks. 

 The light-shunning habit on the part of their ancestors doubtless 

 accounts for the cave-inhabiting blindfishes of to-day. Some 

 diurnal fishes habitually stay in the shade of some tree, or log, or 

 pier, while others are found in the open. There seems to be a 

 complete gradation between the blindfishes, which always live in 



FIG. 1546. Hog Sucker, Catostomus nigricans Le Sueur. Actual size, 81 mm. long. 



total darkness, and those fishes, like some sunfishes, that live in 

 total light, as far as this exists. 



The eye is not the only light-perceiving organ of aquatic verte- 

 brates. The skin is sensitive to light in many cases. The blind- 

 fishes, whose eyes are not functional and may be entirely removed, 

 nevertheless appreciate the difference between light and dark. 

 The young after having their eyes removed are as sensitive to 



