64 



MICROSCOPIC ILLUSTRATIONS. 



of valves is a pair of singular- looking- appendages, a, a. 

 They are probably nervous ganglions, auxiliary to the 

 motion of the vessel, but so exceedingly pellucid as 

 scarcely to be defined. The action of the valves is a 

 most interesting and beautiful sight. While in their 

 greatest state of collapse, the point of the lower valve is 

 seen closely compressed within the upper one. At the 

 commencement of the expansion of the artery, the blood 

 is seen flowing in from the lateral apertures, (as shewn 

 by the arrows) and at the same time the stream in the 

 artery commences its ascent. When it has nearly 

 attained its greatest state of expansion, the sides of the 

 lower valve are forced upwards by the increasing flow 

 of the blood from the section below the valve, the lateral 

 openings are closed, and the main current of the blood 

 is projected through the two valves." 



A portion of the larva of the Ephemera, greatly magnified, displaying the 

 structure of the Circulating System. 



f< It is not easy to see this beautiful structure of the 

 valves of the great dorsal vessel, for it is only when the 

 insect is in a state of exhaustion, or has been just so 

 much compressed as to destroy voluntary motion without 

 entirely depriving it of life, that it is possible to subject it 



