PHYSIOLOGY OF THE INVERTEBRATA. 3 



unit is also a physiological unit, and tlie multi-cellular 

 mass is strictly a compound organism, made up of a multi- 

 tude of physiologically independent cells. The physiological 

 activities manifested by the complex whole i*epresent the sum, 

 or rather the resultant, of the separate and independent physio- 

 logical activities resident in each of the simpler constituents 

 of that whole. 



•' The morphological changes which the cells undergo in 

 the course of the further development of the organism do 

 not affect their individuality ; and, notwithstanding the 

 moditicatiou and conHuence of its constituent cells, the adult 

 organism, however complex, is still an aggregate of morpho- 

 logical units. Nor is it less an aggregate of physiological 

 units, each of which retains its fundamental independence, 

 though that independence becomes restricted in various 

 ways. 



•' Each cell, or that element of a tissue which proceeds 

 from the modification of a cell, must needs retain its susten- 

 tative functions so long as it grows or maintains a condition 

 of equilibrium ; but the most completely metamorphosed 

 cells show no trace of the generative function, and many 

 exhibit no correlative functions. Contrariwise, those cells 

 of the adult organism which are the unmetamorphosed 

 derivatives of the germ exhibit all the primary functions, 

 not only nourishing themselves and growing, but multiplying 

 and frequently showing more or less marked movements." 



The cell theor}^ first ably worked out by Schwann, has led 

 physiology, aided by chemical means, to scrutinise more 

 profoundly the mechanism of the vital acts ; it has taught it 

 to refer them to their ultimate agents — that is, to the histo- 

 logical elements themselves, which vary in function and in 

 form in complex beings, and which we must consider as 

 playing a part in the mechanism of organised beings 

 analogous to that of atoms in chemical molecules. 



In the lowest animals all functions are performed by all 

 tissues : the sarcode of an amoeba assimilates, l^reathes. 



