CHAPTER IV 



CLASSIFICATION OF ANIMALS ACCORDING TO THE 

 MOST GENERAL CHARACTERISTICS OF THE 



ENVIRONMENT 



The oldest attempts to classify animals, as in the biblical account 

 of the creation, are based upon their habitat. Pliny in the first century 

 a.d. divided animals into Aquatilia, Terrestria, and Volatilia. This 

 ecological classification has its justification, independent of morpho- 

 logical taxonomy, in view of the obvious structural adaptations in- 

 volved. The primary division of an ecologic classification of animals 

 is into terrestrial and aquatic forms, and internal parasites, whose 

 conditions of life somewhat resemble those of aquatic animals and will 

 not be discussed here in detail. 



All animals whose bodies are surrounded by air, and not water, 

 belong to the first class, i.e., not only the animals living on the earth's 

 surface, but also the subterranean and wood-boring forms, and the 

 flying groups (insects, birds, and bats), all of which are subjected to 

 the influence of the air with its low density, high oxygen content, and 

 varying humidity. It must be admitted that there are various inter- 

 mediates between these two groups. The common frogs in the northern 

 hemisphere are air-breathers in summer, but strictly aquatic during 

 their winter hibernation. Many newts are aquatic in summer and 

 hibernate on land. The earthworm may be entirely immersed in water 

 in the event of continued rains, and some earthworm genera, Allurus 

 for example, are true aquatic animals. 



Among aquatic animals one may distinguish between the primarily 

 aquatic forms, whose ancestors have always been aquatic, and sec- 

 ondarily aquatic animals which have reentered the water from the 

 land. It is true that the terrestrial ancestors of the secondarily aquatic 

 animals, in their turn, were derived from aquatic forms, but not all 

 the characteristics associated with terrestrial life are lost in the return 

 to the aquatic habitat; some are retained by the secondarily aquatic 

 animals as an unmistakable record of their racial history. The pri- 

 marily aquatic animals, with which we are immediately concerned, 

 obtain their oxygen from the dissolved supply in the water. All are 

 poikilothermal. The gr ea t, density of the aqueous medium helps to 



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