206. THE MET AZO A — DIVISION OF LABOR 



Functions Common to All Animals. — The same general functions 

 performed by a single cell are performed by a many-celled animal. 

 But in the Metazoa the various functions of the single cell are taken 

 up by the organs. In a complex organism, like man, the organs 

 and the functions they perform may be briefly given as follows : — ■ 



(1) The organs of food taking: food may be taken in by indi- 

 vidual cells, as those lining the pores of the sponge, or definite 

 parts of a food tube may be set apart for this purpose, as the mouth 

 and parts which place food in the mouth. 



(2) The organs of digestion: the food tube and collections of 

 cells which form the glands connected with it. The enzymes in the 

 fluids secreted by the latter change the foods from a solid form 

 (usually insoluble) to that of a fluid. Such fluid may then pass by 

 osmosis through the walls of the food tube into the blood. 



(3) The organs of circulation : the tubes through which the blood, 

 bearing its organic foods and oxygen, reaches the tissues of the body. 

 In simple forms of Metazoa, as the sponge and hydra, no such 

 organs are needed, the fluid food passing from cell to cell by osmosis. 



(4) The organs of respiration: the organs in which the blood 

 receives oxygen and gives up carbon dioxide. The outer layer of 

 the body serves this purpose in very simple animals ; gills or lungs 

 are developed in more complex animals. 



(5) The organs of excretion : such as the kidneys and skin, which 

 pass off nitrogenous and other waste matters from the body. 



(6) The organs of locomotion: muscles and their attachments 

 and connectives; namely, tendons, ligaments, and bones. 



(7) The organs of nervous control: the central nervous system, 

 which has control of co5rdinated movement. This consists of 

 scattered cells in low forms of life; such cells are collected into 

 groups and connected with each other in higher animals. 



(8) The sense organs : collections of cells having to do with the 

 .reception of sight, hearing, smell, taste, and touch. 



(9) The organs of reproduction: the sperm and egg-forming 

 glands. 



Almost all animals have the functions mentioned above. In most, 

 the various organs mentioned are more or less developed, although 

 in the simpler forms of animal life some of the organs mentioned 

 above are either very poorly developed or entirely lacking. 



