INVERTEBRATA, 



CHAPTER I. 



GENERAL CHARACTERS OF ANIMALS. 



Physical Conditions of Life. An animal, chemi- 

 cally considered, consists of a few elements 1 united 

 with each other into extremely unstable combinations, 

 which are at every moment undergoing chemical 

 change. The constituent materials are constantly 

 becoming grouped into more simple and stable com- 

 pounds, and in that state they are either eliminated 

 cr retained in an inactive condition, while other 

 materials from without are being taken in, and becom- 

 ing so modified that they replace the molecules re- 

 moved by the previous decomposition. As long as 

 life lasts, these conditions of waste and repair continue ; 

 so that the particles of the bodies of all animals 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, with smaller 

 quantities of sulphur and phosphorus. 

 B 



