THE ALLIES OF THE PERCH: PISCES 



321 



The absence of coloring matter on the side which is in con- 

 tact with the bottom seems to be due to the fact that little 

 or no light reaches it, for when a number of young floun- 

 ders were placed, under experimentation by Professor Cun- 

 ningham of England, so that the under side was illuminated 



by a mirror for four months, 

 nearly all the specimens devel- 

 oped pigment on the skin of 

 that surface. 



The sunfishes are interesting on account of 

 their nest-building habits. They scoop out a space, 

 sometimes three feet across, in the clear sand or 

 gravel in shallow water. Here the eggs are laid, 

 f i^j anc ^ ^ nen g uar( l e d ty both parents, their pug- 

 \ lit? nacity keeping larger fishes at a distance. 



VlfL^ 5 ^ Lung-Fishes. The lung-fishes (Fig. 160), or 

 Dip'noi (Gr. di, two ; pneo, breathe), are fishes 

 with an almost entirely cartilaginous skeleton, 

 and the body is covered with scales instead 

 of with tubercles. The gills are covered by an 

 operculum, and the tail tapers to a point. The 

 structural character of the greatest interest is. 

 suggested in their common name; in place of 

 the air-bladder there is a true lung, or pair 

 of lungs, opening from the ventral side of the 

 alimentary canal. Dipnoans differ from other 



fisheS ' t0 ' in the fact that the heart is incom " 

 pletely divided into three chambers. These ani- 



mals are interesting as the possible ancestors 

 of the toads, frogs, and their allies, which we shall consider 

 later. Though numerous in earlier times, there are but three 

 genera now in existence, one each in the rivers of Queens- 

 land (in Australia), Brazil, and tropical Africa. " The 

 Australian species inhabits rivers which at certain seasons 



Dean) 



Aftfr 



