102 A NATURALIST IN THE GREAT LAKES REGION 



is maintained, by means of which all live and move and have 

 their being. The simple plants and animals can absorb the 

 oxygen and give off the carbon dioxide, together with other 

 waste products of combustion, directly through the moist skin. 

 But the higher types have needed to develop gills or similar 

 structures and some sort of circulation to take the gas to the 

 working cells. The more complex plants growing totally sub- 

 merged have their leaves dissected so that they consist of numer- 

 ous threadlike filaments or else they are long and narrow, 

 ribbon-like, so that every part of the leaf is near the oxygen 

 supply of the water. See, for instance, the leaf of water milfoil, 

 of bladderwort, of hornwort, all common to our ponds or streams 

 (Fig. 64). The water buttercup at times grows submerged, again 

 only partly so. The submerged portions have leaves that are 

 finely dissected, the aerial portions the usual buttercup leaf. It is 

 interesting to note that the animal gill is built on the same general 

 plan as the submerged leaf, a series of filaments, sometimes 

 branched. In the animal these are provided with vessels that take 

 up the needed oxygen and carry it to the distant internal organs. 

 The transition from water-inhabiting animal to land dweller 

 is seen in the life-history of our common toad. The eggs are 

 laid in the water, hatch into tadpoles that are vegetarians, and 

 breathe by means of gills. In time these develop legs, resorb 

 their tails, come out on to the land, replacing the gills by lungs 

 and feeding entirely on insect food. When the first sea worms 

 crawled out of the water to make their burrows on land, a wealth 

 of food was awaiting them in their virgin hunting ground, for 

 the land was largely unoccupied by animal life. When some 

 primitive fish crawled out on the mud banks, using its fins as 

 legs and there is one that does this still it was rewarded by 

 a lot of food which others of its kind could not get. But soon 

 the earth swarmed with life, and even the air supported a full 

 complement. Then, apparently some of the animals that earlier 

 forsook the water for the land or air went back again to hunt in 

 the water, and no more interesting series of adaptations is to 



