AN INTRODUCTION TO THE METAZOA 77 



cally equivalent and having the same functions, e.g. the legs 

 of man and the legs of a bird. 



Many organs are usually necessary for the performance of a 

 single function; for example, the proper digestion of food in a 

 complex animal requires a large number of organs collectively 

 known as the alimentary canal and its appendages. These 

 organs constitute the digestive system. Similarly, other sets of 

 organs are associated for carrying on other functions. The 

 principal systems of organs and their chief functions are as fol- 

 lows : — 



(1) Digestive system — Digestion and absorption of food. 



(2) Circulatory system — Transportation of food, oxygen, and 

 waste products. 



(3) Respiratory system — Taking in oxygen and giving off 

 carbon dioxide. 



(4) Excretory system — Elimination of waste products of 

 metabolism. 



(5) Muscular system — Motion and locomotion. 



(6) Skeletal system — Protection and support. 



(7) Nervous system — Sensation and correlation. 



(8) Reproductive system — Reproduction. 



It has been shown in Chapter II that the Protozoa carry on 

 the processes of digestion, respiration, excretion, etc., without 

 the presence of definite organs. Likewise many of the simpler 

 Metazoa do not have special organs for the performance of cer- 

 tain functions, but the more complex animals are provided with 

 well-developed systems of organs. The following paragraphs give 

 a general account of the systems of organs and their functions in 

 complex animals. 



(1) The digestive system has for its functions the changing 

 of solid food into liquids and the absorption of these liquids into 

 the blood. This system consists usually of a tube, the alimentary 

 canal, with an opening at either end of the body. Connected 

 with this tube are a number of glands. Solids taken in as food 

 are usually broken up in the mouth, where they are mixed with 



