VII MOLLUSGA—THE CIRCULATORY SYSTEM 205 



cavity, to enter the auricle. The vessels of the respiratory network form projecting 

 ribs on the surface of tlie mantle. The pallial epithelium in the mantle cavity is 

 ciliated. 



The efferent pulmonary vessels, which, near the kidney, run along the right side 

 of the pulmonary vein, first enter the kidney and lireak into a fine vascular network 

 before passing into that vein. 



Tlie cephalic aorta does not jtass through the oesophageal ring, but runs between 

 the pedal and visceral ganglia ; this is said to be the case in most Opisthobranchia. 



In Oiristhopncumonic Pulmonata {e.g. Daudebardia, Testacella), in which the 

 small or rudimentary visceral dome has shifted to the posterior end of the body, and 

 the organs elsewhere found in the dome (liver and genital organs) now lie in the body 

 cavity above the foot, and thus in front of the posteriorly i)laced heart, the posterior 

 aorta (A. visceralis) is much reduced, but the anterior aorta (A. cephalica) is strongly 

 develoj^ed. The po.sterior aorta supplies only the posterior lobes of the liver and the 

 hermajihrodite gland, and the anterior aorta (cephalic aorta, A. asceudens) has thus 

 to supply the anterior lobes and even part of the genital organs, which usually receive 

 their blood from the posterior aorta. 



In Oncidium, there is an arteria visceralis eorresjionding with the posterior 

 aorta, which branches ofi' soon after the aorta leaves the heart, but it here runs 

 anteriorly. 



(c) Opisthobranchia. — Here also the arrangement is essentially the same as in 

 the Prosobranchia, though modified by the different position of the gills, as has been 

 already briefly noted. 



Gastropteron affords a good illustration of the circulatory system of the Tedi- 

 brancMa. The heart, which is enclosed in a spacious pericardium, lies to the right, 

 in front of and above the base of the gill. It lies transversely, the larger and more 

 nuiscular ventricle to the left, the auricle to the right. Out of the ventricle springs 

 the aorta, which at once divides into a i>osterior and an anterior aorta. The anterior 

 aorta enters the cephalic cavity, giving off as its princij)al arteries : (1) the artery of 

 the copulatory organ. (2) The two lai-ge pedal arteries, each of which again soon 

 divides into two branches, viz. (a) an anterior artery, which branches richly in the 

 parapodia ; (&) a posterior artery, which runs back on each side parallel to the 

 median line of the foot. (-3) The arteries of the cephalic disc. (4) The arteries 

 of the oesophageal bulb and of the cesoj)hagus. (5) The anterior end of the aorta 

 itself branches in the tissues surrounding the mouth. The following are the chief 

 branches of the posterior aorta : (1) The gastric artery. (2) The hepatic arteries. 

 (3) The genital arteries. The venous blood flows back from all parts of the body 

 through richly - branched channels into two large venous sinuses, one of which 

 represents the cephalic and the other the body cavity. Wide but short vessels 

 convey the venous blood out of these sinuses into the kidney, which contains a 

 rich venous lacunar system. From the kidney it flows direct into the afferent 

 branchial vessel, becomes arterial in the gills, and collects in the efferent branchial 

 vessel, which, as the branchial vein, soon enters the auricle. 



All the venous blood in Gastropteron, therefore, on its way back to the heart, 

 jiasses first thi'ough the kidney and then tlirough the gill, so that only arterial blood 

 flows through the heart. 



This is, however, not by any means the case in all Tedibranchia. For e-xamjile. 

 in Pleurobrmickus, a large part of the venous blood passes from a dorsal circular 

 sinus tlirough a very short but wide pa.ssage dii-ect into the branchial vein close to 

 its point of entrance into the auricle, passing by both the kidney and tiie gill. 



Dorididse. — Without going into details as to the circulatory system of this group, 

 it may be mentioned that part of the venous blood passes directly through two 

 lateral vessels into the auricle. Another ])art flows into an inner venous circumanal 



