CIRCULATORY SYSTEM IN .VRACHNIDA 169 



The aorta itself continues as a considerable vessel under the 

 brain until it reaches the third neuromere of the suboesophageal 

 ganglionic mass. Here the aortic arch around the oesophagus 

 is formed. The arch is very short and connects the aorta with 

 the right and left thoracic sinus opposite the base of the second 

 pedal artery as shown in figures 1 and 2. Two pairs of vessels 

 issue from the aortic arch. The first pair are the large cephahc 

 arteries shown in figiu^e 1. The second pair cannot be shown 

 to advantage in the planes chosen for our drawings and is there- 

 fore not represented there. But these vessels are nevertheless 

 constant in their position and easily found. Their roots are 

 in the right and left anterior inner edge of the aortic arch, near 

 its junction with the sinus and ahnost directly above the third 

 vessel connecting the sinus with the supraneural artery. The 

 two vessels run convergingly upward, feeding the wall of the 

 oesophagus. 



Each thoracic sinus gives rise to small and thin vessels con- 

 necting the sinus with the supraneural artery, and to four large 

 vessels for the appendages. Of these vessels the first is the 

 largest and sphts almost immediately into two branches. The 

 outer branch (fig. 1 and 2, 1, AP) is the first pedal artery, while 

 the inner, longer, and stouter branch is the pedipalpal artery 

 {APP). The latter gives off a thin branch directed inwardly, 

 following in its course the curvature of the ganglionic mass, 

 and connecting with the supraneural artery just behind the 

 pharynx. A branch of this supphes the tissues in front of the 

 pharynx. 



Posteriorly, the right and left sinus merge with each other and 

 form a connection with the supraneural artery. This artery 

 is single and runs in the median line above the ventral nervous 

 system and closely applied to it. Anteriorly it runs to the very 

 end of the ganglionic mass, turning downward in its course and 

 now continuing backward in the midventral line below the gan- 

 ghonic mass as subneural artery. Posterior to the thoracic sinus 

 the supraneural artery continues as a straight vessel in the median 

 line above the nervous system and closely appHed to it through 

 the entire abdomen. I have not followed its course in the post- 

 abdomen. 



