430 EVENINGS AT THE MIGKOSCOPE. 



CHAPTEK XX. 



PROTOZOA AND SPONGES. 



We are so accustomed to see certain of the vital 

 functions of animals performed by special organs or 

 tissues, that we wonder when we find creatures which 

 move without limbs, contract witliout muscles, respire 

 without lungs or gills, and digest without a stomach or 

 intestines. But thus we are taught that the function is 

 independent of the organ, and, as it were, prior to it ; 

 though in nine hundred and ninety-nine cases out of a 

 thousand it be associated with it. In truth, the simplest 

 forms of animal life display very little of that division 

 of labour, the minuteness of which increases as we as- 

 cend the organic scale ; the common tissue is not 

 yet differentiated (to use the awkward term which 

 is becoming fashionable among physiologists) into 

 organs, but is endowed with the power of fulfilling 

 various offices, and performing many functions. In all 

 probability, the function is but imperfectly performed ; 

 the specialization of certain tissues, and their union into 

 organs, and the complexity of such combinations, 

 no doubt, perform the given function in a far more 

 complete degree ; and it is the number and elaborate- 

 ness of these that constitute one animal higher in the 

 scale than another. The human lung is no doubt a 

 more complete breathing apparatus than the entire 



