378 SE3N STRTJCTUflE A^'D EXCRETION 



make the body difficult to grasp, and at the same tune 

 moisten the outer layer so as to permit exchange of carbon 

 dioxide and oxygen. Claws, spines, fur, hoofs, horns, etc., 

 are all modifications of the horny outer part of the epidermis, 

 and each arises from a dermal papilla, or papillse, just as 

 do the hair and nails of man. The dermal layer of animals 

 is often much thicker than that of man, and when tanned 

 it forms what we call leather. 



Excretory Organs of the Lower Antmat.s. 



Aside from the use of the skin as an excretory organ, 

 there are no structures comparable to kidneys in anj- of the 

 lower animals until we reach the worms and mollusks. 

 The protozoans carry on the removal of wastes hj simply 

 throwing out the wastes of metabolism into the surround- 

 ing water or liquid in which they hve. As the animals 

 become larger, the cells multiplj^ in numbers, and it becomes 

 more difficult for each cell to get rid of its ■wastes. If this 

 waste thrown out by the cells remains in the body it may 

 produce serious harm to the tissues. The solution of this 

 difficulty is met in the development of glands whose func- 

 tion is to take the collected wastes of the cells from the 

 circulatory system, to transform these wastes into a secre- 

 tion of uniform character, and then remove this secretion 

 to the outside of the body. 



All such glands, of whatever character or structure, are 

 identical in plan ^\^th the air sac of the lung, the sweat 

 gland of the skin, and the kidney of man. In all, the cells 

 which do the secreting are placed in such relation to the 

 blood system that the body wastes (carbon dioxide, water, 

 or urea) may be readily absorbed, and when transformed 



