ENVIRONMENTAL CLASSIFICATION 41 



brackish water, and this would affect the eggs as above outlined. Ac- 

 cording to this supposition, the only animals which could persist in 

 fresh water would be those whose eggs react to reduced salinity as do 

 those of Palaemonetes. Animals may, however, be transported directly 

 into fresh water, as happens with Dreissena, which is carried by ships; 

 and the yolk content of the eggs of such species can have no sig- 

 nificance for the process of acclimatization. 



The eggs of fresh-water animals, like the animals themselves, re- 

 quire some protection against dilution and swelling by the inward 

 diffusion of water. Marine forms may have completely naked eggs. 

 Fresh-water animals have eggs with a dense covering, like those of 

 Hydra or the river crayfish; or with a gelatinous covering, like those 

 of snails and frogs; or their eggs may be enclosed in a thick-walled 

 case, as in the planarians. 



Terrestrial animals. — Terrestrial animals contrast with the aqua- 

 tic forms in many ways. The total inhabitable space available for 

 terrestrial life is much smaller than that at the disposal of aquatic 

 forms. The surface of the oceans and inland waters combined amounts 

 to about 362,250,000 sq. km., whereas the total land surface is only 

 147,650,000 sq. km. The oceans, with an average depth of 3681 meters, 

 so far as investigated, are everywhere habitable and inhabited by 

 living organisms in one stratum above the other from bottom to 

 surface. On land, ice and desert are nearly or quite closed to life, 

 and as animals are unable to raise themselves permanently in the air, 

 they are confined to a single layer, which even in favorable cases, as 

 in tropical rain-forest, is only 25-70 meters deep. In spite of these 

 spatial restrictions, four-fifths of the known species of animals are 

 terrestrial. 



Terrestrial life offers advantages which result in the luxuriance of 

 such animals as are able to adapt themselves to it. The most impor- 

 tant of these advantages is the abundance of available oxygen. Less 

 than 7 cc. of oxygen are usually dissolved in a liter of water, while a 

 liter of air contains 207 cc. Owing to the role played by oxygen in the 

 release of chemical energy, this makes possible an enormously in- 

 creased rate of combustion for air-breathers if a sufficient food supply 

 is provided. Terrestrial animals accordingly live much more inten- 

 sively than the primary aquatic forms. Such muscular activity as that 

 of the wing muscles of insects, amounting to 330 contractions per 

 second in the common housefly, is unknown among water-breathers, 

 although extraordinary muscular efficiency is attained by the pelagic 

 fishes. 



Many different aquatic animals have at different times become 



