PHYSIOLOGY OF THE INVERTEBRATA. g 



Metazoa. And notwithstanding the multiplicity of its parts, 

 no Echinoderm is so highly differentiated as a physiological 



machine as is a snail It is not mere multiplication of 



organs which constitutes physiological differentiation; but 

 the multiplication of organs of different functions in the 

 first place, and the degree in which they are co-ordinated, 

 so as to work a common end, in the. second place. Thus, 

 a lobster is a higher animal, from a physiological point of 

 view, than a Cyclops, not because it has more distinguishable 

 organs, but because these organs are so modified as to per- 

 form a much greater variety of functions, while they are all 

 co-ordinated towards the mstintenance of the animal by its 

 well-developed nervous system and sense-organs. But it is 

 impossible to say that, e.g., the Arthropoda, as a whole, are 

 physiologically higher than the Molhisca, inasmuch as the 

 simplest embodiments of the common plan of the Arthropoda 

 are less differentiated, physiologically, than the great majority 

 of Mollusks." (Huxley.) 



