360 GENERAL ZOOLOGY 



The organ, by means of which oxygen is absorbed from the 

 water and carbon dioxide is excreted into it, is some sort of gill, 

 the fundamental structure of which is the same for both inverte- 

 brate and vertebrate aquatic animals. Very small forms lack 

 gills and are capable of respiring through the body surface. 

 Aquatic mammals, such as the whale, seal, porpoise, and manatee, 

 have taken to an aquatic existence secondarily and obtain oxygen 

 from the air above the water. The same is true of aquatic 

 reptiles and some aquatic insects. 



Locomotion is simplified for an aquatic animal because the 

 density of water is great enough to support partially or completely 

 the body weight. Movement is accomplished by pushing against 

 the water, and resistance to forward motion is reduced by the 

 shape of the body. It is interesting that aquatic mammals have 

 evolved a fish-shaped body with degenerated limbs. 



Land. — Terrestrial animals are enveloped by air from which 

 oxygen is absorbed by means of lungs, the buccal cavity, the 

 surface of the body, or by a tracheal system. Locomotion over 

 the ground is accomplished as a rule by means of limbs, which 

 in many cases raise the body clear of the ground. The weight of 

 the body, whether carried or dragged, is a factor that does not 

 figure in aquatic locomotion. There is therefore a correspond- 

 ingly greater energy requirement involved in terrestrial as 

 compared with aquatic locomotion which is reflected in the 

 greater oxygen (and food) requirement of the terrestrial animals 

 as a whole. Both oxygen and food requirements are increased 

 for flying animals since the density of air is very much less than 

 that of the body. The requirements for flying are met by a 

 heightened basal metabolism and, particularly in the larger 

 flying forms such as birds, by a lightening of the body. The 

 form of the body also is such as to reduce friction to a minimum 

 during flight. The streamlining of both fishes and birds has the 

 same significance. 



Animal Communities. — Animals whose needs are met by 

 similar conditions, or who have become adapted to similar 

 conditions, form an animal community. The members of the 

 same community, though often unlike and belonging to widely 

 separated taxonomic groups, are capable of living together 

 because their responses to the environment are similar. Less 

 specialized and therefore less rigidly adapted forms may live in 



