100 How Animals Changed 



tinually have a favorable medium for metabolic activities. An ani- 

 mal with a hard or impervious external covering has special advan- 

 tages on land — the chief colonizers have been arthropods, snails, 

 and vertebrates. The permeability of a tadpole's skin decreases 

 when it metamorphoses into a frog. The skin becomes a better 

 protective mechanism for impeding the passage of solutes. On land 

 it must also retard desiccation. This usually does not facilitate res- 

 piration, and breathing in land animals is usually carried on in in- 

 ternal cavities where respiratory membranes may be kept more or 

 less moist without much loss of water. Furthermore, terrestrial am- 

 phibians can survive a greater loss of body water than aquatic types 

 (Thorson & Svihla, 1943). 



Respiration 



The percentage of oxygen in the earth's atmosphere is greater 

 and more constant than in natural waters. This compensates to 

 some extent for the dangers that land animals endure from desicca- 

 tion and variable temperatures. It is also true that respiration for 

 an aquatic animal is easier in saline than non-saline water (Prenant, 

 1929) . This has probably impeded the migration of certain marine 

 types into fresh water. 



Baldwin (1937) gives an excellent discussion of the changes in 

 respiration when animals left the ocean and took up Ufe in fresh 

 water and on land. Respiration must supply oxygen and also get 

 rid of carbon dioxide. "Considerable modifications in respiratory 

 organs became necessary when animals left an aquatic in favor of a 

 terrestrial environment. The advantages to be gained thereby in- 

 cluded the exchange of a relatively poor oxygen supply for a very 

 rich one. But respiration has always remained essentially aquatic. 

 The respiratory epithelium, whether it be that of a gill or a lung, is 

 always covered by a stationary aqueous layer. . . . The relative in- 

 efficiency of the respiratory organ has been made good by an in- 

 crease in the area of the respiratory epithelium per unit of body 



