246 REVERSAL OF THE CURRENT. 



between the outer surface of the sac and the interior 

 surface of the body; for many take this course, on 

 both sides of the sac. 



But to return to the current which passes up the 

 right side : arriving at the upper angle of the body, 

 the stream turns off to the left abruptly, principally 

 passing along a fold or groove in the exterior of the 

 breathing sac, until it reaches the left side, down 

 which it passes, and along the bottom, until it arrives 

 at the entrance of the heart, and rushes in to fill the 

 vacuum produced by the expansion of its walls after 

 the periodic contraction. This is the perfect circle ; 

 but the minor streams that had forked off sideways in 

 the course, as those within the sac, for example, find 

 their way to the entrance of the heart by shorter and 

 more irregular courses. 



One or two things connected with this circulatory 

 system are worthy of special notice. The first 

 is that its direction is not constant but reversible. 

 After watching this course followed with regularity 

 for perhaps a hundred pulsations or so, all of a sud- 

 den, the heart ceased to beat, and all the globules 

 rested in their circling course, that I had su]3posed 

 incessant. Oh, ho ! said I, — 



" Thy stone, O Sisyphus, stands still, 

 Ixion rests upon his wheel ; — " 



when, after a pause of two or three seconds, the pul- 

 sation began again, but at the opposite end of the 

 heart, and proceeded with perfect regularity, just as 

 before, hut in the opposite direction. The globules, 

 of course, obeyed the new impulse, entered at their 



