100 PROCEEDINGS OP THE AMERICAN ACADEMY 



distended with blood. Only at considerable intervals of time does the 

 plexus in the head become entirely filled, and usually then for only a 

 fraction of a second ; so that it is difficult to map out the entire plexus 

 in any one animal, especially as those animals that have the most vigor- 

 ous circulation are also the ones that move most rapidly. But, from 

 an examination of a large number of specimens, I am certain that the 

 arrangement in the head region is as follows : the dorsal vessel passes 

 forward to above the brain (PI. I. fig. 6) and there bifurcates. The 

 branches resulting from its bifurcation pass downward at the sides of 

 the brain, and, bending backward, finally unite at a considerable dis- 

 tance behind the mouth to form the ventral vessel. Before bifurcating, 

 however, it gives off three pairs of lateral branches. Of these three 

 pairs, the most anterior is given off just behind the mouth (the posi- 

 tion of which is indicated in the figure as though seen through the 

 tissues of the head). Each one of this pair, after passing outward to 

 near the margin of the head, bifurcates, one of its branches passing 

 forward and the other backward, and both joining the recurrent branch 

 of the same side which results from the bifurcation of the dorsal vessel. 

 The other two pairs of branches given ofT by the dorsal vessel pass out 

 laterally, and probably also join the recurrent branches just mentioned. 

 Some specimens show many other smaller vessels, and there is always 

 a more or less complicated anastomosis of vessels around the pharynx, 

 joining the dorsal and ventral vessels. It is possible that all these 

 vessels present individual variations of branching and arrangement. 



In the posterior region (PI. II. fig. 5) the ventral vessel passes 

 backward to near the notch between the two lobes on the ventral bor- 

 der of the pavilion. There it bifurcates, one branch running to the 

 right and the other to the left along the border of the pavilion. 

 These two branches become united on the dorsal border of the pavil- 

 ion, and thus form a comjilete ring around its margin. Each half of 

 this marginal ring gives rise to two branches. Each of the smaller 

 pair arises close to the ventral bifurcation, and, passing along the me- 

 dian edge of the small lobe on the ventral border of the pavilion, 

 makes a curve outward in that lobe, and then passes forward to unite 

 with the larger branch lying on the same side. The larger branches 

 arise at nearly opposite points in the lateral portion of the marginal 

 ring, and, quickly converging as they pass forward, unite in the median 

 dorsal line to form the dorsal vessel. This form of plexus differs from 

 that found in Dero ohtusa only to the extent of the modification ren- 

 dered necessary by the difference in the form of the pavilion. 



The contractile branches uniting the dorsal and ventral vessels in 



