376 EVENINGS AT THE MICROSCOPE. 



CHAPTER XX. 



PEOTOZOA 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 without 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 ascend the organic scale ; the 

 common tissue is not yet differentiated* (to use the 

 awkward term which is becoming fashionable among physi- 

 ologists) into organs, but it is endowed with the power 

 of fulfilling various offices, and performing many functions. 

 In all probability, the function is but imperfectly performed; 

 the specialisation 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 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,f and the human eye sees more perfectly than 



* The term is used to express the cellular development of an organi- 

 sation from its original germ. 



t The name given to a class of minute animals, from their being 

 generally developed in infusions of animal or vegetable matter. 



