8O J. B. JOHNSTON. 



continued along the vessel for about a third or half its length 



O 



These muscle fibers in the dorsal portion of the parietals produce 

 the active pulsations which have been described in an earlier 

 paper (loc. cit., p. 323). 



The walls of the hearts have the same structure as that of the 

 dorsal vessel, except that they are covered with chloragogue 

 cells only in their dorsal portion, elsewhere by peritoneum. The 

 circular muscle fibers are large and the layer somewhat stronger 

 than that in the dorsal vessel. 



The structure of the vessels determines whether they shall pro- 

 pel the blood by their pulsations or only carry it, and the ac- 

 count of the structure accords with the well-known facts con- 

 cerning the pulsations of the vessels. Pulsations in the dorsal, 

 parietals and hearts are well established ; pulsations in other 

 vessels, described by Harrington, have not been seen by the author 

 and to whatever extent they occur they must be produced with- 

 out muscle fibers. 



Valves are present in the dorsal vessel and in all the vessels 

 connected with it, namely, the dorso-intestinals, dorso-typhloso- 

 lars, parietals, lateral oesophageals (?) and hearts. The valves 

 in the dorsal are a pair of large thick flaps attached to the lateral 

 walls of the vessel at a point a short distance behind each septum 

 and immediately behind the openings cf the parietals. These 

 valves are always directed forward and allow the free passage 

 of blood during the pulse wave. As the contraction wave ap- 

 proaches, the valves are brought into contact and at the moment 

 of greatest constriction the two flaps are tightly pressed together 

 and completely close the lumen of the vessel. The efficiency of 

 the valves is secured and increased by a considerable thickening 

 of the circular muscle layer at the valve (Fig. 2). The valves 

 do not act in the ordinary manner of flap valves, but the two 

 fleshy flaps are pressed together and form a large mass which 

 fills the vessel. In the region of the hearts a pair of valves is 

 found in the dorsal vessel a short distance in front of each pair 

 of hearts. 



The valves in the dorso-intestinal, dorso typhlosolar and pari- 

 etal vessels are essentially the same in form and position. In 

 each of these vessels (Fig. 2) a pair of small fleshy flaps are sit- 



