CIECULATORY SYSTEM IN .■UlACHNIDA 167 



In the dorsal midline of the heart, partly imbedded in a groove 

 in the wall of the heart, the cardiac nerve extends from one end 

 of the heart to the other (text figure B, CN). The nerve is 

 clearly visible in all transverse sections and unquestionably 

 corresponds to the cardiac nerve described in Chilopoda 

 (Duboscq), Protracheata, and other Arthropoda. As my mate- 

 rial is not specially prepared for the study of nerves, I am unable 

 to find a connection of the nerve with the brain, but such 

 connection has been described by Pohce in Euscorpius. 



The structure of the anterior aortic valve is best understood 

 from median sagittal sections and sections which traverse the 

 valve more or less at right angles. In the first (fig. 1, AV) 

 the valve appears as a line attached to the dprsal wall of the heart 

 exactly under the epicardiac ligaments, inclined downward, and 

 about two and a half times as long as the diameter of the heart at 

 the place of the attachment of the valve. In reahty the valve is 

 a muscular membrane arising from the dorsal half of the wall of 

 of the heart and attached to the sides of the vessel throughout its 

 length. The anterior edge of the valve is longer than the diameter 

 of the vessel. The valve has, therefore, a pecuUar shape, being 

 concave or troughhke at its free edge and convex or arched at 

 its base. About half-way between its base and end the valve 

 is drawn tight in the equator of the transverse section of the 

 vessel. Such a section is represented in figure 4, which also 

 shows that the valve is not a fold, but consists of a single layer 

 of transverse muscular fibers with elongated nuclei. There is 

 always a greater accumulation of blood-cells above the valve than 

 below it, showing that the action of the valve is perfect. 



The structure of the posterior aortic valve is more difficult 

 to ascertain, and is somewhat different from the anterior one. 

 Text figure A represents the position of the posterior valve as 

 being not far from the posterior edge of the seventh abdominal 

 tergite. This position is constant in specimens of all ages. The 

 valve seems to have the shape of a cone, the open free apex of 

 which is directed posteriorly, while the broad base is attached 

 to the wall of the heart along its entire circumference. This 

 valve, too, has a single layer of circular muscle fibers composing it, 



