18 STUDIES IN BIOLOGY. 



left side till near the stomach, when it turns down under the 

 stomach, and its final branches follow and supply the coil of the 

 cardiac csecurn. It is the gastro-intestinal artery. 



The two aortas, anterior and posterior, the two anterior pallial 

 arteries (not in magellanicus) , and one (M. latus) or more (M. eclulis) 

 gastro-intestinal arteries are the main vessels leaving the aortic bulb. 

 I shall now take each of these in turn, and give its course and 

 branches in greater detail than could be done in the above general 

 account. 



Anterior aorta in M. latus. The anterior aorta before it enters 

 the supra-sesophageal cavity gives off some small branches to the 

 liver. After entering the supra-sesophageal cavity it gives off 

 numerous branches, so that the walls of this cavity are covered with 

 the fine ramifications of vessels containing arterial blood. As above 

 mentioned, a strong artery is given off from the aorta on the right 

 side (Figs. .21 and 24, A) just at the posterior dorsal end of the cavity. 

 This artery passes round the right wall of the cavity, branching on its 

 route till near the anterior retractor. The other chief artery in this 

 cavity (tr a, Figs. 24 and 27) is given off from the aorta a little 

 behind the left anterior retractor, and lies in the floor of the cavity. 

 From the anterior side of this branch, which lies horizontally trans- 

 verse to the long axis of the body, are given off the arteries that 

 supply both sides of the outer labial palps and the dorsal side of the 

 inner, labial palps. The ventral side of the inner labial palps is 

 supplied from another source (Figs. 27 and 28) . 



From the lower side of the transverse artery just mentioned a 

 strong branch turns downward just at the left side of the floor of this 

 cavity (Fig. 21) . It might be quite as correct to say that the aorta 

 gives off a strong artery just behind the anterior retractor, one 

 branch of which forms a transverse artery in the floor of the supra- 

 sesophageal cavity, and the other passes almost directly downward 

 (a v a, Fig. 24). In any case, the downward branch passes to the 

 ventral side of the body between the anterior retractors, turning 

 backwards horizontally when it reaches the lower side of the anterior 

 retractors ; it runs back in the gutter or groove between these 

 muscles, rising to their upper side when near the foot. It passes 

 then between the retractors of the foot and the middle retractors of 

 the byssus, going under the pedal ganglion, and ends between the 

 bases of the retractor muscles in many fine branches or capillaries. 

 I shall refer to this artery as the anterior ventral artery. Just at 

 the point where the anterior ventral artery turns backward it sends 

 forward a small artery in the median line (t a, Figs. 24 and 28), 



