SUBK1NGDOM METAZOA. 41 



OHAPTEE III. 



SUBKINGDOM METAZOA. 



THE Metazoa are equivalent to colonies of Protozoa, the 

 individual cells of which have differentiated in various direc- 

 tions, some being more especially contractile, others nutritive, 

 others irritable, others reproductive, etc., instead of each one 

 for itself performing equally all the functions necessary for 

 existence. A physiological division of labor of a more or less 

 perfect kind is introduced among the individuals composing 

 the colony, and the welfare of each individual becomes de- 

 pendent upon the proper performance by its colleagues of 

 their special functions ; in short, the individualities of the 

 component cells are merged in the higher individuality of 

 the whole organism. 



Physiologically a Metazoon is equivalent to a Protozoon, 

 but morphologically it is the equivalent of a large number of 

 them. Each is physiologically an individual, but morpholog- 

 ically the Metazoon is a colony of Protozoan individuals. To 

 harmonize the physiological and morphological conceptions 

 of an individual it is necessary to recognize several grades 

 of morphological individuality of which the cell may be as- 

 sumed to be the lowest. In the Metazoa the physiological 

 differentiations of the cell-individuals are accompanied by 

 structural differentiations, so that it is possible, as a rule, to 

 determine from its structure what the function of a cell may 

 be ; aggregates of similar cells are termed tissues or tissue-indi- 

 viduals, and as the simplest Metazoa are complexes of various 

 tissues, such a complex forms the third grade of individu- 

 ality and may be termed an Organ-individual. A complex of 

 organ-individuals united to form a physiological unit consti- 

 tutes an individual of the third grade, the Metamere-individual, 

 while the fourth grade, the Cormus, is formed by a similar 

 union of a number of metameres, as, for instance, in the 

 Earthworm, each joint or segment of which is a rnetamere. 



