CHAPTER XVII. 



Complex Life — The Invertebrates. 



\yf ORE than half way up the tree of life, measured In 

 terms of geologic time, vertebrate life came Into being. 

 This upper part of the tree of animal life consists of five 

 classes, namely — Fishes, Amphibia, Reptiles, Birds and 

 Mammals. All these classes have the same basic structure In 

 their organization. The more Important characteristics 

 which cause them to be grouped under one heading are a 

 jointed vertebral column composed of cartilage or bone 

 which acts as a protection and support of the nervous sys- 

 tem. From now on up the scale of life, the nerve sensations 

 play the major part In animal progress. To the backbone 

 there are usually attached pairs of ribs, two pairs of limbs, 

 which may be either fins, legs or wings, and In the front, this 

 backbone terminates In a more or less highly developed skull 

 as a protective covering for the directing brain. In the body 

 cavity protected by the ribs, a digestive system is located 

 which turns the energy force of food Into body energy. A 

 highly organized circulatory system Is also enclosed within 

 the body cavity which carries the blood to all parts of the 

 body. 



The function of breathing in the aquatic animals Is car- 

 ried on by means of gills and in the air breathing animals 

 by means of lungs. The nervous system centering In the 

 brain and through the spinal cord controls the activities of 



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