The Nature of Animal Life. 



is slight; in many cases, however, the form alters very 

 markedly during the successive stages of the life of the 

 individual, as is seen in the frog, which begins life as a 

 tadpole, and perhaps even more conspicuously in the 

 butterfly, which passes through a caterpillar and a chry- 

 salis stage. Still, these changes are always the same for 

 the same kind of animal. So that we may say, each 

 animal has a definite form and shape or series of shapes. 



2. Animals breathe. The essential thing here is that 

 oxygen is taken in by the organism, and carbonic acid gas 

 is produced by the organism. No animal can carry on its 

 life-processes unless certain chemical changes take place 

 in the substance of which it is composed. And for these 

 chemical changes oxygen is essential. The products of 

 these changes, the most familiar of which are carbonic 

 acid gas and urea, must be got rid of by the process of 

 excretion. Respiration and excretion are therefore essential 

 and characteristic life-processes of all animals. 



In us, and in all air-breathing vertebrates, there are 

 special organs set apart for respiration and excretion 

 of carbonic acid gas. These are 

 the lungs. A great number of 

 insects also breathe air, but in a 

 different way. They have no lungs, 

 but they respire by means of a 

 number of apertures in their sides, 

 and these open into a system of u / c 

 delicate branching tubes which 

 ramify throughout the body. Many 



Organisms, however, SUCh as fish Fig. 1. Diagram of spiracles 

 .. , ., and air-tubes (tracheae) of 



and lobsters and molluscs, breathe an insec t (cockroach), 

 the air dissolved in the water in }H ^^ l f^ l (S^^^ff^ 

 which they live. The special organs *" a d yo ,f ^^he^ir^ubes ( ar e c repre- 

 developed for this purpose are the 8 ^ e ^ r JS b S5f to STriJht 

 gills. They are freely exposed to of the figure. 

 the water from which they abstract the air dissolved therein. 

 Wheli the air dissolved in the water is used up, they sicken 

 and die. There can be nothing more cruel than to keep 



