10 TEXT-BOOK OF ZOOLOGY 



food which fail to be absorbed, and are consequently useless as nourish- 

 ment, are separated in the form of excrement, or faeces. 



8. Organs of Secretion (or Excretion).— As in the combustion of 

 wood ash is left, so also in the combustion of animal bodies materials 

 remain behind which are not only valueless, but actually noxious. These 

 accordingly must be removed from the body. We have already become 

 acquainted with one of these products, carbonic acid gas, and its separa- 

 tion by means of the lungs. There are, however, others which also 

 require removal. Here again the blood plays the role of an intermediary 

 agent. It takes up these waste products of assimilation in the capillaries, 

 conveying them to the kidneys, and in most mammals to the skin. In 

 both these organs they are separated in the form of urine and sweat 

 respectively. (For the importance of sweat in another relation see 

 under "Dog.") 



9. The Skin of vertebrates consists (with one single exception, the 

 Lancelet) always of two layers, the epidermis and the derma. 



