416 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



surface of the roots. But in all animals a part at least 

 of the exchange is through an infolded surface, or a bay 

 or harbor. In the lowest animals {Protozoa), absorp- 

 tion or imports are through an infolded surface or bay, 

 the stomach ; but elimination or exports are still through 

 a directly external surface, an exposed coast line. In 

 the highest animals all exchange, both imports and ex- 

 ports, are through an infolded surface. 



In going up the scale of life there are three progres- 

 sive changes in this regard, i. The relegation of more 

 and more of exchange to an infolded surface or bay 

 until allis thus relegated. 2. The gradual differentiation 

 of the several kinds of functions, both absorptive and 

 eliminative (which in the lowest animals are performed 

 in every part alike), and their localization, each in 

 its own separate place. 3. The increasing complexity 

 of the infolding until it becomes almost inconceivably 

 great. If the simple infolding may be compared to a 

 bay or harbor, then the infolding of this again may be 

 compared to the slips and docks of the harbor. 



Now, every such surface through which exchange 

 takes place, as already said (page 20), no matter how 

 complex, is covered with a pavement of living nucleated 

 epithelial cells, through the agency of which the ex- 

 change takes place. Such a complexly infolded surface, 

 covered with epithelial cells webbed together by con- 

 nective tissue and invested and isolated by fibrous or 

 serous membrane, constitutes an organ absorptive or 

 eliminative. The extreme complexity is especially char- 

 acteristic of eliminative organs, and among these the most 

 complex of all is the lungs. 



Secretion versus Excretion.— Now, elimination 

 and eliminative organs are of two kinds — viz., secretions 

 and excretions, secretory organs and excretory organs. 

 In the former the products do not pre-exist in the blood, 



