116 FOUNDATIONS OF BIOLOGY 



son with the plant is reflected again in the fact that the body 

 of the higher plants is essentially a combination of a series 

 of tissue systems and organs, each of which plays a particular 

 part in the economy of the whole, while that of the higher 

 animal is a cooperating series of organs, or ORGAN SYSTEMS. 

 The organ systems of animals may be classified as the 



INTEGUMENTARY AND SKELETAL SYSTEMS which Constitute the 



covering and the framework of the individual; the ALIMEN- 

 TARY, RESPIRATORY, CIRCULATORY, and EXCRETORY SYSTEMS 



which directly or indirectly are concerned with nutrition; 

 the NERVOUS SYSTEM which, in cooperation with the system 

 of SENSE ORGANS, the MUSCULAR SYSTEM, etc., not only coordi- 

 nates the various parts of the individual, but also orients the 

 whole with respect to its environment; and, finally, the RE- 

 PRODUCTIVE SYSTEM which makes possible the continuation 

 of the race. The fundamental life processes for which 

 these systems provide must be carried on by all animals, and 

 the chief differences in the structure of animals, from the 

 lowest to the highest, is a resultant of the means adopted to 

 serve these essential functions under different exigencies im- 

 posed by the environment and mode of life. 



A. THE CHIEF GROUPS OF ANIMALS 



The animal kingdom may be divided into two main 

 groups; on the one hand, the unicellular animals, or PROTOZOA, 

 comprising about ten thousand known kinds, nearly all 

 of which are microscopic, such as Amoeba, Paramecium, and 

 their -allies; and on the other hand, multicellular forms, or 

 METAZOA. The latter division includes animals ranging in 

 size from those which are so small that hundreds can sport 

 in a drop of water, to the present-day Whales and the Dino- 

 saurs of the past. Although the actual stages in the transi- 

 tion from the Protozoa to the Metazoa are unknown, among 



