CHAPTEK XIII 

 VEKTEBKATA 



Irritability. — In vertebrates (e.g. fish, birds, elephants, 

 human beings) as in all other animals the physiological 

 manifestations are those that distinguish the simplest 

 forms of living matter, irritability, assimilation, repro- 

 duction, and in this group as in all the others the spe- 

 cial method of manifestation depends on the structure. 



Vertebrates are distinguished from other animals by 

 the possession of an internal skeleton with a spinal col- 

 umn. This furnishes the framework for the body and 

 helps to preserve the form of the animal. The bones are 

 capable of co-ordinated movement through the irrita- 

 bility of the attached muscles. When a muscle con- 

 tracts it becomes shorter and thicker; its two ends ap- 

 proach and the bones to which it is attached approach. 

 The stimulus which moves the muscle is usually carried 

 to it by nerves which are very sensitive. They conduct 

 a stimulus more quickly than any other form of proto- 

 plasm, and through their intervention the muscles act 

 more quickly than they would otherwise act. 



Assimilation. — Assimilation takes place in vertebrates 

 as in all other animals, but the alimentary canal and its 

 appendages are more highly developed. For food they 

 need carbohydrates, fats, proteids, water and salts, 



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