INTRODUCTION TO THE METAZOA 



87 



which secretes secretins and receives pan- 

 creatic fluid from the pancreas and bile 

 from the Hver. Absorption occurs in the 

 small intestine, and both digestion and ab- 

 sorption continue in the large intestine. Un- 

 digested material and other wastes are elimi- 

 nated by the large intestine. 



Circulatory system 

 (transportation of food, 

 oxygen, and waste products) 



Blood has many functions. It carries 

 oxygen from the lungs to the tissues, food 

 material to the tissues, hormones and other 

 internal secretions to various parts of the 

 body, and metabolic wastes to the excretor}' 

 organs; it maintains a normal temperature 

 in warm-blooded animals, and aids in main- 

 taining an internal fluid pressure. The heart 

 receives blood from the veins and forces it 

 through the arteries. Arteries carry blood to 

 the tissues, and veins carry it away from the 

 tissues. Lymphatic ducts and lymphatic 

 capillaries carry lymph. Tissue fluid (fluid 

 surrounding cells) transports nourishment 

 from the blood to the tissues, and metabolic 

 wastes from the tissues to the blood. 



Respiratory system (taking 

 in oxygen and eliminating 

 carbon dioxide) 



Air enters the respiratory system by way 

 of the mouth or nostrils, passing through 

 the larynx, which contains the vocal cords; 

 then on through the trachea, and the bron- 

 chial tubes, into the lungs, where external 

 respiration takes place; here the blood gains 

 about 8 per cent of oxygen and loses about 

 7 per cent of its carbon dioxide. Internal 

 respiration involves passage of oxygen from 

 the blood to the tissue fluid and thence into 

 the tissues, and the passage of carbon diox- 

 ide from the tissues to the tissue fluid and 

 thence to the blood. 



Excretory system 

 (elimination of waste 

 products of metabolism) 



The kidneys extract urine from the blood; 

 urine consists largely of water and urea. 

 Urine passes through the ureters into the 

 bladder, which acts as a storage reservoir, 

 and from the bladder to the outside through 

 the urethra. The lungs, digestive tract, and 

 skin also serve as excretory organs. 



Muscular system (motion 

 and locomotion) 



Muscles receive stimuli and respond to 

 them and are capable of contraction and 

 recovery. Striated skeletal muscles operate 

 the bones and produce motion. Smooth 

 visceral muscles bring about movements of 

 the viscera. Cardiac muscles are responsible 

 for the beating of the heart. 



Skeletal system (support, 

 protection, attachment) 



All vertebrates and many other animals 

 have firm frameworks or skeletons that give 

 support and protection to the bodies and 

 may provide places for attachment of mus 

 cles. 



Nervous system (sensation, 

 conduction, and correlation) 



The nervous system enables an animal to 

 become aware of its environment, to see, 

 hear, smell, taste, and feel. It correlates the 

 different parts of the body, exerts control 

 over the internal organs, and is responsible 

 for human thought and conduct. Tlie cen- 

 tral nervous system consists of the brain and 

 spinal cord. The peripheral nervous system 

 comprises the organs of special sense and the 

 nerves connecting the central nervous sys- 

 tem with various parts of the body; these 



