THE LUNGFISH 83 



sorbed and for a few days the fish gains markedly in 

 weight and may acquire a swollen, waterlogged appear- 

 ance. That renal function is quickly re-estabhshed is 

 shown, however, by the excretion of sulfate, which is not 

 excreted by the gills. Then within ten days or so the 

 animal, although emaciated in the extreme, is back in 

 excretor>' balance and ready to start life anew. 



Surprisingly, Protopterus breeds shortly after it 

 emerges from estivation so that it has scant time for 

 recuperation. The breeding fish make a nest of sorts in 

 the swamp grass in shallow water where the eggs, ex- 

 ternally fertilized, are deposited on the bottom. The 

 young have a suctorial organ resembling that seen in 

 tadpoles, by means of which they attach themselves to 

 the sides of the nest, and they have long external, 

 cutaneous gills— both of these larval organs disappearing 

 as the mature state, which ushers in aerial respiration, is 

 reached. Until the eggs are hatched, which requires 

 about eight days, and while the larvae remain in the 

 nest, the male stays on guard and lashes the water with 

 his tail to improve aeration. However, in his ravenous 

 hunger, he may eat all the young. 



Air breathing was probably practiced by all the im- 

 portant groups of Devonian fishes except the elasmo- 

 branchs, but how many of them undertook estivation 

 is unknown. It is only in recent years that lungfish 

 burrows have been recognized in the Paleozoic sedi- 

 ments of Texas, a few containing the remains of a fish 

 that failed to escape its prison in the mud. We may be- 

 Heve, however, that by means of their aerial respiration 

 the lungfishes survived in swamps fouled with deca3dng 

 plants, and on occasion in wet mud for short periods, 

 and that their cousins the Crossopterygii, by virtue of 

 their primitive lungs, squirmed across the land from one 

 pool to another, to lay the foundations for the evolution 

 of the first fotur-footed, air-breathing animals. 



