422 EVENINGS AT THE MICROSCOPE 



CHAPTER XX 



PROTOZOA AND SPONGES 



WE are so accustomed to see certain of the vital 

 functions of animals performed by special or- 

 gans or tissues, th'at we wonder when we find 

 creatures which move without limbs, contract without mus- 

 cles, 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 labor, the minuteness of which increases as we 

 ascend 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 en- 

 dowed with the power of fulfilling various offices, and 

 performing many functions. In all probability, the func- 

 tion is but imperfectly performed; the specialization of 

 certain tissues, and their union into organs, and the com- 

 plexity of such combinations, no doubt, perform the given 

 function in a far more complete degree; and it is the num- 

 ber and elaborateness 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 

 ciliated surface of an Infusory, and the human eye sees 

 more perfectly than the loose aggregation of pigment gran- 



