158 ANIMAL BIOLOGY 



osmosis and be carried to the cells 1 of the body. When the 

 digested food reaches the cells, it may be used by the proto- 

 plasm either in making more living matter or, as we shall 

 now see, for the release of energy. 



115. Respiration and the production of energy. In our 

 laboratory study we watched the movements of the gill 

 bailer and saw that it caused a current of water to enter 

 the posterior end of the gill chamber and flow over the gills, 



thus bringing oxy- 

 gen to the filaments 

 (Fig. 111). The 



gin baiier \ thin-walled blood 



vessels in the fila- 

 ments absorb the 

 oxygen, and the 

 blood then passes 



FIG. 111. Gills of a crayfish. 



on into other blood 



vessels, which carry it back to the heart, whence it is forced 

 all over the body of the crayfish, and so the oxygen in the 

 blood passes into the cells as does the food. Now what 

 becomes of the oxygen ? 



As in plants (P. B., 80), the oxygen unites with ele- 

 ments in the foods and protoplasm of the cells and produces 

 oxidation and liberation of energy, which gives the crayfish 

 the power to contract its muscles and so push against the 

 water with its abdomen and tail fin, thus propelling the animal 

 backward, or to open its nippers and shut them and so se- 

 cure food. In fact, all the work that the crayfish performs 

 is made possible through the burning of its foods or proto- 

 plasm by the oxygen. 



1 We have shown in plant biology (41) that plants consist of cells 

 which are largely composed of living matter known as protoplasm. 

 This is also true of animals (126). 



