VII 



MOLLUSGA—THE CIRCULATORY SYSTEM 



209 



angles to it. The heart of the Myopsidce occupies a position halfway between those 

 just mentioned. 



The heart here described is the arterial heart, which corresponds with the heart 

 of the other Mollusca. It is called arterial to distinguish it from the venous hearts, 

 wliieh will be described below. 



Circulation. — It is important to note that the circulatory system is at least 

 partially closed. There is not only a richly-branched arterial, but a richly-branched 

 venous system, the vessels of which have walls of their own. These two systems 

 pass into one another in certain parts of the body, e.g. the integument and certain 

 muscle layers, through a system of capillary vessels. In other parts, however, the 

 arterial branches conduct the blood into a lacunar system ; when it has become 



Fig. 174.— Circulatory system, venous appendages of tlie uepbridial system, and gills of 

 Sepia officinalis, anterior view (after Hunter). 1, Aorta cephalica ; 2, ctenidiuni ; 3, vein leading 

 to the ctenidium : 4, branchial heart ; 5, appendage of the branchial heart (pericardial gland) ; 

 ti, venous appendages of the nephridial system ; 7, aorta abdominalis ; 8, Aona abdominalis ; 9, 

 lateral veins ; 10, vena cephalica ; 11, auricles ; 12, ventricle (c/. Fig. ISG). 



venous, the blood collects out of this into sinuses (especially into a peripharyngeal 

 cej)halic sinus), and flows to the gills through veins with walls of their own. 



Two aorta rise from the ventricle: (1) the aorta cephalica, which runs downward 

 (upwards in the figure) to the head, and (2) the aorta abdominalis, which runs up 

 towards the apex of the visceral dome. The former is much stronger than the latter. 

 The aorta cephalica first gives off branches to the mantle and to the anterior wall of 

 the body, and then provides the stomach, the pancreas, the digestive gland, the 

 resophagus, the salivary glands, and the funnel with arteries. After accompanying 

 the oesophagus, it divides in the head into two branches, which run to the bases of 

 the arms, and there break up into as many arterise brachiales as there are arms. 



The aorta abdominalis supplies with arteries the liind-gut, the ink-bag, the 

 genital organs, the dorsal part of the body wall, and the fins, when these latter are 

 present. 



Only in the Oegopsidce are the aorta limited to the two, above described, springing 

 from the heart. In the Odopoda and the Mycrpsida;, there are other arteries rising out 

 of the ventricle, and running to the same part of the body as the aorta abdominalis 

 VOL. II P 



