PHYSIOLOGY OF THE INVERTEBRATA, 193 



the heart, which is a tubular organ running along the back of 

 the insect, and hence called the dorsal vessel (Fig. 40). This 



is formed of a series of sacs opening one into the other, from 

 behind forwards, in such a manner that the folds formed by 

 the junction of the sacs serve as valves to prevent the reflux 



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