214 LECTURE V. 



lating organs and lungs like those of vermes. 

 Lyonnet, Cuvier, and others, assert that the 

 dorsal vessel found in insects gives off no 

 branches, and that no motion of fluids can 

 be perceived in them, even by the aid of 

 the microscope. Professor Cuvier says, 

 " tout le corps est nourri par une fluide 

 stagnant." Yet even vessels can produce 

 a free and forcible motion of their contents, 

 as is evident in vegetables, and in the ab- 

 sorbents of the higher classes of animals. 

 If there were no current in the fluids of 

 insects, how could liquors run so freely 

 as they are known to do from the contorted 

 tubes that form their glands ? Professor 

 Cuvier is disposed to consider the irrita- 

 bility of animals as greatly dependent on 

 the aeration of the blood ; and ascribes 

 the vivacity and power of insects to its 

 great exposure to air. Yet this opinion 

 cannot, I think, be maintained as a general 

 inference in physiology: for what animals 

 are more agile, powerful, and indefati- 

 gable than fish, and yet we have reason 

 to believe their blood is very imperfectly 

 aerated ? 



