PHYSIOLOGY OF THE INVERTEBRATA. 



MSilAZOA.— continued. 



If animals are looked upon as machines for doing work, 

 they differ from one another in the extent to which this 

 work is subdivided. " Bach subordinate group of actions or 

 functions is allotted to a particular portion of the body, 

 which thus becomes the organ of those functions ; and the 

 extent to which this division of physiological labour is 

 carried differs in degree within the limits of each common 

 plan, and is the chief cause of the diversity in the working 

 out of the common plan of a group exhibited by its members. 

 Moreover, there are certain types which never attain the same 

 degree of physiological differentiation as others do. 



" Thus, some of the Protozoa attain a grade of physiological 

 complexity as high as that which is reached by the lower 



