182 HUGH WATSON. 



hei-maphrodite gland^ to which it also supplies blood. In A . 

 burnupi, but not in the other species, this branch also gives 

 off a small artery to the hind end of the right division of the 

 liver. In A. parva the posterior aorta divides immediately 

 into three branches at the point where it separates from the 

 anterior aorta, but in the other species the division of the 

 vessel takes place further forward. No posterior aorta is 

 present in A. dimidia, and there is probably none in A. 

 purcelli either. 



I would suggest that perhaps this so-called posterior aorta 

 may be merely the left hepatic artery which has become dis- 

 placed, and that the Gastropoda (with the possible excep- 

 tions of Haliotis and Fissurella^) resemble the Amphi- 

 neura and the more primitive members of the Pelecypoda 

 in having no true posterior aorta. The posterior position of 

 this artery would be accounted for by the posterior position 

 which the left division of the liver has come to occupy owing 

 to the torsion of the visceral hump. Its comparatively large 

 size in most genera might be due to the fact that the left 

 division of the liver is generally much larger than the right, 

 since it extends up the spire of the shell. And as the herma- 

 phrodite gland is usually more or less embedded in the left 

 division of the liver, it would be likely to be supplied with 

 blood by the same artery. This view will be made clear by 

 the accompanying diagrams a to d. Diagram a represents a 

 primitive symmetrical arrangement, and c the condition after 

 the torsion has taken place; B shows a hypothetical stage 

 between a and c, while d represents the most usual arrange- 

 ment of the arteries and liver in Apera. 



The anterior aorta continues forward on the right side of 

 the liver, crossing over the intestine in A. gibbonsi, A. 

 T)iiriiupi, and A. sexangula. In A. gibbonsi, A. 

 dimidia, and probably in A. purcelli, it gives off a single 

 artery to the liver on reaching the level of the anterior loop 



' See Wegmann, H., 'Arch. Zool. Exper.' (2nd ser.), 1884, vol. ii, pp. 

 352, 353, pi. xviii, figs. 1-4 ; and Boutan, L.. ibid., 1885, ex vol. iii bis 

 suppl., pp. 34, 130, pi. xxxiii, figs. 5, 6. 



