DIVISION OF LABOR 181 



many-celled animal. But in the many-celled animals the various 

 functions of the single cell are taken up by the organs. In a com- 

 plex organism, like man, the organs and the functions they per- 

 form 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 animals, 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 coordinated 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 organs of sense: collections of cells having to do with 

 the reception and transmission of sight, hearing, smell, taste, touch, 

 pressure, and temperature sensations. 



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

 organs. 



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 



