144 A COURSE ON ZOOLOGY. 



with the atmosphere ; now, as the blood does not circu- 

 late, as insects have no lungs nor gills to bring the vital 

 fluid in contact with the air, this contact must take 

 place throughout the whole body of the creature wher- 

 ever the blood may be, and this is what actually occurs. 

 The segments that compose the external envelope are 

 pierced laterally with small holes called stigmata, and 

 these open into little tubes called tracheae, which dis- 

 tribute the air throughout the interior of the organism 



FIG. 78. 



CIRCULATION OF AN INSECT. a, c, c, dorsal vessel. The arrows indicate the 

 direction of the blood. 



by dividing up into small branches of exceeding fine- 

 ness that penetrate into all the organs. The walls of 

 the tracheae are supported by a coiled fibre, and so are 

 always kept open for the free circulation of air. 



The nervous system is ganglionic, and nothing need 

 be said concerning it more than has been given in Chap- 

 ter XIII. 



To conclude this brief description of insect organiza- 

 tion, we must consider one fact that we have noticed as 

 an exception in the division of vertebrates, and which ac- 

 quires a remarkable generality in the articulates. This 

 is metamorphosis. 



Insects lay eggs, usually in large numbers, and these 



