THE MULTICELLULAR ANIMAL 65 



And finally, within the body — though perhaps not of the body 

 — is the germinal tissue destined to contribute not to the in- 

 dividual but to the propagation of the race during reproduction. 



So the multicellular animal — the organism as a whole — is a 

 multitude of cooperating protoplasmic units: upward of one hun- 

 dred thousand billions in Man. However, we must not overlook 

 the whole which is greater than its parts. The cells merge their 

 individuality in that of the entire organism, and as long as its cells 

 remain associated they are to be regarded, not as individuals, but 

 as specialized centers of action and reaction of the living body 

 itself, by means of which physiological division of labor is made 

 possible. 



C. Organs and Organ Systems 



Since the similar cell components of multicellular organisms 

 are grouped to form tissues, it follows that the major working 

 units, or organs, of the animal body as a whole are formed of 

 tissues. In other words, an organ is a complex of tissues which 

 has assumed a definite form for the performance of a certain func- 

 tion — a major division of the body which allows the tissues and, 

 therefore, the cells devoted to a special function to play their 

 part under the most suitable relations to internal or external con- 

 ditions: for example, the human hand composed of bone, muscle, 

 nerve, etc. (Figs. 34, 125.) 



As one would naturally expect, among the lowest Metazoa there 

 are forms in which the body is relatively simple, without highly 

 specialized tissues and organs, but in most animals specialization 

 is carried still another step forward by the grouping of organs de- 

 voted to the performance of some one general function into an 



ORGAN SYSTEM. 



The organ systems may be classified as the integumentary 

 and supporting systems which constitute the covering and the 

 framework of the individual; the alimentary, respiratory, cir- 

 culatory, 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 sys- 

 tem, etc., not only coordinates the various parts of the individual, 

 but also orients the whole with respect to its environment; and, 

 finally, the reproductive system which makes possible the con- 

 tinuation of the race. The fundamental life processes for which 



