METAZOAN ORGANIZATION 59 



A system is an aggregation of organs properly associated and 

 related to perform some general function of life. There are ten 

 different systems usually recognized : 



a. The Integumentary System is composed of the skin and its out- 

 growths, such as hair, nails, scales-, horns, hoofs, and similar struc- 

 tures. Its principal purposes are protection, primarily, with some 

 degree of excretion and respiration, some absorption, and regulation 

 of body temperature. 



b. The Skeletal System composes the supporting framework of the 

 body. The bony and cartilaginous tissues make up the material of 

 this system. The vertebral column, skull, ribs, sternum, and bones 

 of the limbs are the general parts of the vertebrate skeleton, and 

 they serve for the support of the body as a whole and for the pro- 

 tection of the internal, vital organs. 



c. The Muscular System consists of muscles, the voluntary, stri- 

 ated group moves skeletal parts and accomplishes locomotion; the 

 nonstriated, involuntary group is concerned with the movements of 

 the internal organs (viscera), and the cardiac muscle produces the 

 heart action. 



d. The Digestive System of the higher animals includes the mouth, 

 pharynx, esophagus, stomach, small intestine, large intestine, and ac- 

 cessory glands. The general form of the system is that of a tube, and 

 it is frequently called the alimentary canal. The functions of in- 

 gestion, digestion, egestion, absorption, secretion, and very little ex- 

 cretion are performed by this system. In general, it puts the food 

 in solution so that it may be absorbed by the blood. 



e. The Respiratory System consists of structures capable of de- 

 livering oxygen to the body and eliminating carbon dioxide. In some 

 forms the general surface of the body serves the purpose, but in all 

 higher forms there are special structures for this function. Tracheae 

 are found in insects, gills of various modifications in many aquatic 

 Metazoa, and lungs in the terrestrial vertebrate forms; accessory to 

 the lungs are the nasal passages, pharynx, larynx, trachea, and bronchi. 



f . The Circulatory or Vascular System is a very extensive one con- 

 sisting of the heart, arteries, veins, capillaries, lymph spaces, lymph 

 nodes, and lymphoid glands. The general functions are: (1) to dis- 

 tribute blood carrying food, oxygen, and hormones from glands of 

 internal secretion to the tissues; (2) to collect and transport to the 

 point of exit carbon dioxide, liquid wastes, bacteria, and other foreign 

 matter. 



