24 STUDIES IN BIOLOGY. 



Arteries to the stomach, intestine, and liver. The stomach and 

 the parts of the intestine anterior to the pericardium are supplied 

 chiefly by the large gastro-intestinal artery (g a, Fig. 24) , whose 

 course has already been fully described. Its branches supply the left 

 side of the pyloric caecum, the anterior coil of the intestine, the 

 underside of the stomach, and the cardiac ceecum. Branches of this 

 artery are sent out into the liver in all directions, and the main 

 central mass of the liver is supplied by it. The superficial portions of 

 the liver are supplied partly by branches from the large arteries near, 

 as from the aorta, anterior pallial arteries, or from the anterior 

 ventral artery. The right side of the pyloric csecum in front of the 

 aortic bulb and the portions of the liver and intestine near the aortic 

 bulb are supplied by branches, from the base of the gastro-intestinal 

 artery, or from the aortic bulb. The portions of the alimentary 

 canal lying in the floor of the pericardium are supplied by the branches 

 of the posterior aorta. The underside of the rectum at the anterior 

 and also at the posterior end of the pericardium receives a small 

 artery ; from the upper side of the posterior aorta at the latter point 

 (Fig. 24, D), and from the base of the gastro-intestinal artery or the 

 outside of the aortic bulb at the former point. The whole course of 

 the alimentary canal is covered by a very fine system of capillaries. 



Comparison of the Arterial Systems or M. edulis and M. 

 latus. 



I have not been able to give time to work out the whole arterial 

 system of M. magellanicus, and therefore cannot speak of it except 

 with regard to its chief arteries. Of these, its anterior aorta and 

 pallial arteries have been above described, and its posterior aorta 

 resembles that of M. edulis. 



It may be stated as the general rule that in M. latus a single 

 unpaired artery is made to take the place of paired arteries in M. 

 edulis ; that is, the tendency in M. latus is to have a single median 

 vessel where M. edulis has two parallel vessels a little to the sides of 

 the median plane. The arterial system of M. latus is single and 

 median ; that of M. edulis double and equilateral. The point of this 

 comparison will best be understood by referring to the general sketch 

 of the arterial system of M. latus (Fig. 21). Here, taking the 

 anterior aorta and the posterior aorta as one, and overlooking- the 

 divergence of the anterior aorta, it is seen that the arterial system 

 has a main, single median trunk dividing at the anterior and posterior 

 ends only when the separation of the mantle - lobes compels this 

 division. The gastro-intestinal artery, which is not shown in Fig. 21, 



