CHAPTER LXI 



ANIMAL ORGANISMS 



In the chapters which have preceded the phyla of the animal king- 

 dom have been reviewed. In connection with some of these phyla 

 general phenomena were described and certain principles presented. 

 When the animal kingdom is viewed as a whole, however, many con- 

 ceptions are at once suggested which are related to the more general 

 divisions of zoology. In the chapters which follow these will be briefly 

 discussed. 



455. The Organism. — The word organism has been previously used 

 but its meaning has nowhere been clearly stated. When reference is 

 made to such a type as a colonial hydroid or a tapeworm (Fig. 78), and 

 particularly to the Portuguese man-of-war (Fig. 64), the word animal 

 is somewhat equivocal. What is apparently an animal is really an 

 assemblage of many animals. Also during the regeneration of a frag- 

 ment from which develops a complete organism it is a matter of opinion 

 as to when the term animal should be first applied. Likewise, during the 

 development of an egg cell it is a matter of judgment as to when the 

 use of the word animal becomes proper. Since it is clear that the applica- 

 tion of the word animal is attended with a considerable degree of uncer- 

 tainty, it seems desirable to employ another term which may be so 

 defined as to be capable of application to any living thing. No word 

 seems more appropriate than organism. 



456. Definition. — In the sense indicated above an organism may be 

 defined as a mass of living matter capable of maintaining individual exist- 

 ence, and in which all parts contribute more or less to the activities of 

 the whole. This definition covers the one-celled and many-celled ani- 

 mals, the colony, the regenerating fragment, and the individual animal 

 in any stage of development from the egg to the adult. Referring to an 

 organism as a mass of protoplasm need not cause confusion with the defi- 

 nition of a cell, for it should be remembered that if a cell carries on con- 

 tinued independent existence, it is also considered an organism. On the 

 other hand a many-celled animal is a mass of protoplasm divided into 

 cells. In the case of symbiotic forms, whether or not the term organism 

 could be applied to the two taken together would depend upon the 

 degree of cooperation. Thus the green hydra is an organism, though 

 it is composed of an associated plant and animal. In the case of the 

 white ant and a symbiotic protozoan which lives in its alimentary canal, 



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