INVERTEBRATA. 



CHAPTER I. 



GENERAL CHARACTERS OF ANIMALS. 



Physical Conditions of life. An animal, chemically 

 considered, consists of a few elements l united into 

 extremely unstable combinations, which are at every 

 moment undergoing chemical change. The consti- 

 tuent materials are constantly becoming grouped into 

 more simple and stable compounds, and in that state 

 they are either eliminated or retained in an inactive 

 condition, while other materials from without are 

 being taken in, and so modified that they replace the 

 molecules removed by the previous decomposition. 

 As long as life lasts, these conditions of waste and re- 

 pair continue ; so that the particles of the bodies of 

 al lanimals are in a state of constant change. 



The food of animals contains carbon, hydrogen, 

 nitrogen, and oxygen, which must be grouped into 

 complex molecules before the animal can use them 

 for his nutrition. Combinations of the requisite com- 

 plexity can be derived only from previously organised 



1 Carbon, hydrogen, oxygen, and nitrogen. 

 B 



