76 OKHAMANDAL MARINE ZOOLOGY REPORT 



through this all the blood brought by the right auricle passed into the left auricle, and 

 thence into the ventricle by this indirect route. Probably this abnormality indicates 

 a tendency towards the eventual suppression of the right auricle, and to a condition 

 wherein the ventricle will have but a single auricle (the left) opening into it. We 

 can now understand why Placuna has but one aorta (the anterior), whereas in 

 most other Lamellibranchs there are both an anterior and a posterior aortic trunk with 

 independent egress from the ventricle. 



The single aorta {Ao., Plate III, fig. 15) is given off from the ventricle close to 

 the dorsal extremity of its attachment to the visceral mass. It divides almost 

 immediately into two large trunks, (a) an anterior, running dorsally close to the 

 posterior edge of the visceral mass, and (b) a posterior, having a course directed 

 anteriorly and ventrally. 



The anterior aortic trunk, the aorta proper, is markedly asymmetric in its 

 position ; it runs superficially immediately beneath the left surface of the visceral mass 

 and can be readily traced, together with the initial course of its main branches, by 

 removal of the left mantle, when these vessels are seen mapped out clearly on the 

 surface. The first two main branches given ofi" by the aorta after the emergence of the 

 great posterior trunk both run anteriorly and superficially within the visceral mass for 

 a short distance and then turn inwards to supply the stomach, digestive gland and 

 portions of the intestine and gonad [Art.visc. and Art.hep., fig. 15). More dorsal 

 still, a small artery, the posterior common pallial artery {Art.c.p.), leaves the aorta 

 and, first running dorsally, curves round the ventral extremity of the posterior cardinal 

 hinge tooth, and theu, when midway towards the hinge, turns posteriorly and runs 

 along the margin of a thin portion of reproductive tissue to the free dorsal edges of the 

 mantle, where it divides into two long peripheral branches, the right posterior pallial 

 arteries {Art.pp. and pp.'), which run respectively along the posterior edge of each 

 mantle lobe. 



Having given off the posterior common pallial artery, the aorta curves forwards 

 and a little dorsally, and after sending a small vessel to supply the posterior half of 

 the median dorsal hinge lobe at the level of the hinder cardinal hinge tooth and 

 midway between it and the corresponding extremity of the anterior cardinal tooth, the 

 aorta finally ends by bifurcation into a dorsally directed anterior common pallial a,rtery 

 {Art.c.a.) and a ventrally running palpar artery [Art.pa.). The former, after giving 

 off a small artery to the anterior half of the median dorsal lobe, passes forwards to 

 round the ventral extremity of the anterior cardinal tooth and then upwards and 

 forwards to where the mantle lobes join anteriorly. Here the common artery 

 bifurcates in similar manner to its fellow at the posterior end of the hinge, and so 

 gives rise to two branches, the anterior pallial arteries {Art.a.p. and Art.a.p.'), which 

 run marginally within the anterior edges of the respective pallial folds. In this way 

 are formed two circumferential blood-vessels, one in each mantle, which run along the 



