2 Invertebrate*. 



^materials either iiiimal or vegetable. In most vege- 

 tables the forces concerned in assimilation are suffi- 

 ^ q^it , to^ bre^ uj> originally stable compounds, such 

 'as carbonic" acid, ahcTto 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, replacing 

 the losses sustained by each organ in each discharge 

 of its function. 



For the life-processes of animals oxygen is necessary, 

 and special structures, called respiratory or breathing 

 organs, are often provided for taking it in. The car- 

 bonic acid formed from the waste of the tissues is 

 usually 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 fjnctions 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, as they may conveniently be called, 

 usually consists of a mass of protoplasm surrounded 

 by an envelope of some material derived by chemical 

 action from protoplasm. , Ceils continuously grouped 

 make up tissues, and a group of tissues which 



