326 



GENERAL ZOOLOGY 



thus placed the dark surface is protectively colored and the 

 eyes are both on the upper side, which is sometimes the 

 right side and sometimes the left side of the body. When 

 they are hatched they have an eye on either side and they 



swim like other fishes, though 



with a slight leaning to one 



side, which becomes more and 



more marked as they develop, 



till they finally turn entirely over. The eye 



on the light side has meanwhile worked its 



way slowly round to the dark side, where 



it remains with its fellow, looking upward 



when the fish lies on the bottom. 



Care of Eggs and Young. In most fishes the 

 parents take no care of the eggs or young. 

 In some of the fresh-water fishes, such as 

 the sunfishes, for example, the males con- 

 struct a rough nest which is often no more 

 than a hollow in the gravel. Here the fe- 

 male deposits her eggs and the male dis- 

 charges sperm into the water to fertilize 

 them. During development of the eggs the 

 male stands guard to defend them against 

 all intruders, even against the female as 

 well as against outside enemies. 



Lungfishes. The lungfishes (Fig. 168), or 



Dip'noi (Gr. di-, "two" ; pnoe, "breath"), 



Fig. 168. Lungfish 

 After Dean 



in place of the air bladder have a true lung, 

 or pair of lungs, opening from the ventral side of the ali- 

 mentary canal. Dipnoans differ from other fishes, too, in 

 the fact that the heart is incompletely divided into three 

 chambers. These animals 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 



