l6o THE LIVING WORLD. 



make them masters of all the lower forms. Finally, 

 man supersedes them all. 



The Rarity of Terrestrial Life. 



Again we must notice how few are the types 

 of animals that have ever succeeded in adapting 

 themselves to a life on the land. Beyond the 

 vertebrates and the air-breathing articulates (insects, 

 spiders, etc.), there are almost no land animals. 

 Among the mollusks there are a few snails which 

 have become adapted to a thoroughly terrestrial 

 life. There are indeed quite a number of species of 

 snails thus breathing air, but they are all closely 

 allied to each other, and commonly have a much 

 restricted habitat. Evidently, then, only a few 

 mollusks have really been able to acquire the power 

 of breathing air. None of the other invertebrates 

 have been able to acquire the power of living in the 

 air, and we may say, therefore, that there are only 

 two types of animal structure which are adapted to 

 life out of the water. The Protozoa all require water 

 as a medium through which they can feed and carry 

 on respiration. Ccelentera have a body too soft to 

 resist gravity, unless assisted by the buoyancy of the 

 water. The same may be said of the Echinodermata, 

 together with the fact that their characteristic sys- 

 tem of organs, the water system, requires the presence 

 of water to make it of any value. The mollusks are 

 too clumsy, as a rule, to be adapted to terrestrial 

 life. The Vermes mostly respire through the surface 

 of the body or by gills, and either method makes it 

 necessary for them to have the exterior of their body 



