J. B. JOHNSTON. 



FlG. 2. A diagrammatic horizontal section of the dorsal vessel and those emptying 

 into it. One somite and part of a second are shown, and the last part of a pulse 

 wave and the greater part of a following contraction wave are represented. The 

 arrows show the course of the blood and the position and changes of form of the valves 

 are shown. The chloragogue cells covering the walls of the vessels are not drawn. 



