110 TEXTBOOK OF ZOOLOGY 



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) collect and transport to the 

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

 matter. 



g. The Excretory or Urinary System is made up of tubular struc- 

 tures and accessory parts, such as flame cells, nephridia, Malpighian 

 tubules, green glands, and kidneys. In the mammals, the ureters, 

 urinaiy bladder, and urethra are accessory to the kidneys. The kid- 

 neys withdraw liquid waste products of metabolism from the blood 

 and deliver them to the outside of the body. The nitrogenous sub- 

 stances, urea and uric acid dissolved in water, are the principal prod- 

 ucts discharged, 



h. The Endocrine System includes a number of different glands 

 located in various parts of the body. These glands discharge chem- 

 ical substances, known as hormones, directly into the blood. The 

 hormones cooperate to regulate the metabolic activity of the entire 

 body. The thyroid gland of the neck region, adrenals located near 

 the kidneys, and the islands of Langerhans of the pancreas are 

 typical examples of these organs. They go under the names of 

 ductless glands and organs of internal secretion also. 



i. The Nervous System is an organization of the nerve cell bodies 

 and their processes in such a way as to receive stimuli, carry sensa- 

 tions, correlate them, and coordinate the activities of the parts of the 

 body. By the function of the sensory portion of the system, the ani- 

 mal becomes aware of the environment and relates itself to it. In 

 vertebrates the principal parts of the system include the brain, spinal 

 cord, peripheral nerves, autonomic nerves, sense organs, and ganglia. 



