2 Invertebrata. 



materials either animal or vegetable. In most vege- 

 tables the forces concerned in assimilation are suffi- 

 cient to break up originally stable compounds, such 

 as carbonic acid, and to induce the elements to 

 combine into the unstable combinations of which 

 living textures consist. 



The process of repair in animals has three stages, 

 ist, the taking in of material as food; 2nd, the chang- 

 ing of food into a substance capable of forming part 

 of the living organism, i.e. blood ; and 3rd, the laying 

 down of- this assimilated material in the tissues of the 

 body of which it thus becomes a constituent, repairing 

 the waste sustained by each organ in each discharge 

 of its function. 



For the life-processes of animals oxygen is neces- 

 sary, and special structures, called respiratory or breath- 

 ing organs, are provided for taking it in. The carbonic 

 acid formed from the waste of the tissues is got rid of 

 by these organs. 



The material with which the vital properties are 

 connected is of the same nature in all animals and is 

 called protoplasm. The simplest animals are mere 

 masses of this substance, which in them discharges all 

 the functions needful for the maintenance of life ; the 

 more complex are built up of aggregations of particles 

 of the same material, or of substances derived from it 

 in the course of growth. Each of these constituent 

 particles or cells (fig. 9), as they may conveniently be 

 called, usually consists of a mass of protoplasm sur- 

 rounded by an envelope of some material derived by 

 chemical action from protoplasm. Cells continuously 

 grouped make up tissues, and a group of tissues which 



