LIFE PROCESSES OF ANIMALS 363 



ing other substances. In the sponges the cells lining the 

 canals give up their products of excretion to the water and 

 are thereby carried out of the body. Excretion is carried 

 on through the surface of the body wall in the hydra. In 

 the starfish there is a system of tubes containing a fluid 

 (mostly water) that probably acts as an excretory apparatus. 

 Moreover, the respiratory cseca act as organs of excretion. 

 In the earthworm the nephridia and skin get rid of the 

 waste materials, while the tracheae and malpighian vessels 

 are the excretory organs in insects. The nephridia of 

 worms and the malpighian vessels of insects are comparable, 

 in their work, to the kidneys of vertebrates. Respiration 

 is a method of excretion and an exceedingly important life 

 process. If it is completely arrested, death ensues. It is 

 carried on very differently by different animals, as has been 

 shown. Respiration is most highly developed and effec- 

 tually carried on in those animals that breathe by lungs, 

 especially the birds and the mammals. 



Circulation. — The food that an animal eats and digests 

 must be distributed throughout the body in order that the 

 different organs may obtain nutriment for building up worn- 

 out tissues and for the development of energy to accom- 

 plish work. Moreover, the waste material produced in all 

 parts of the body must be brought to the lungs, skin, kid- 

 neys, and other excretory organs to be thrown off. In the 

 higher animals the food is distributed and the waste matter 

 brought to the excretory organs by a fluid (blood) circulat- 

 ing through a system of tubes. The whole process is known 

 as circulation. There is no definite system of blood vessels 

 among the lower animals until the echinoderms are reached. 

 In the Protozoa, the food circulates through the endosarc. 

 The currents of water distribute the food to the different 



