ENVIRONMENTAL CLASSIFICATION 35 



fresh water agree in having the antennal excretory organ larger than 

 in their marine relatives. 6 Overton 7 has shown that water is absorbed 

 by a frog through its skin and that about the same amount is excreted 

 through the kidneys in a given period. No conclusive data on this sub- 

 ject are available for selachians or bony fishes. It appears that the 

 ability to increase the rate of water excretion would enable a marine 

 animal to enter fresh water, and this primary condition for adaptation 

 to fresh water is not equally at the command of all marine animals. 



The ability of the skin to adjust its permeability with respect to 

 the surrounding medium is another method of adaptation in some 

 euryhaline animals. 8 In addition to such permeability changes of the 

 epidermis, a mucous covering of the surface of many aquatic forms, 

 such as the snails and fishes, probably protects them against the entry 

 of water. This is the meaning of the gelatinous covering of buds of 

 aquatic plants. 9 The diatom, Thalassiosira, has a mucilaginous cover- 

 ing which protects it against the varying salt content in the water. 

 The removal of slime from the skin of an eel causes the osmotic 

 pressure of its blood serum to vary with changes in the osmotic 

 pressure of the surrounding water more than in a normal eel. The 

 mucous coat forms an effective barrier to prevent the exchange of 

 water between the outer and inner media. 10 



Of the secondarily aquatic animals, the insects and arachnids with 

 their exoskeleton, and the whales and seals w r ith their horny epidermis, 

 have a protection against the influx of water through the body surface 

 acquired during the terrestrial life of their ancestors. 



Such marine forms as possessed the basic requirements for adapta- 

 tion to life in less saline water at once found favorable opportunities 

 in brackish water, on account of the decreased competition. Rela- 

 tively few brackish-water forms can make the further advance into 

 fresh water. The relatively small number of species of primarily 

 aquatic fresh-water forms shows that not many forms are capable of 

 such adaptation, but a natural capacity for adaptation to fresh water 

 seems to exist in some genera and families, from which representative 

 forms have independently accomplished the transition to fresh water 

 in widely separated regions. Examples of such groups among the bony 

 fishes are the genus Cottus with its relatives; the Gobiidae; the 

 Elopidae; and the Syngnathidae. Among the higher crustaceans the 

 genus Atya (Cape Verde Islands, West Indies, Philippines, and 

 Samoa) and the Palaemonidae (Europe, the Americas, and Africa), 

 may be mentioned. A few important families and even orders of fishes 

 are predominantly inhabitants of fresh waters. 



The echinoderms and tunicates are entirely absent from fresh 



