MOLLUSGA—THE CIRCULATORY SYSTEM 201 



B. Special. 



1. Amphineura. 



a. Chitonidse (Polyplacophora). — The heart is symiiietiical, with two lateral 

 auricles. 



The ventricle and the two auricles are long tubes. The auricles are in open 

 communication with the ventricle about the middle of their length. Besides this, 

 the two auricles pass into one another posteriorly, the posterior end of the ventricle 

 also opening into them at this point. 



The ventricle lies against the dorsal wall of the pericardium, to which it is 

 attached by a median band of endotlielium. The ventricle passes into an aorta 

 which allows the blood to flow into the ea-lom through apertures in its wall. With 

 the exception of the pedal arteries, the rest of the circulatory system is lacunar ; 

 there are no vessels with walls of their own. 



The venous blood is collected from the lacunar system of the body (primary 

 ccelom) into longitudinal channels which run on each side under the pleurovisceral 

 cords. From these channels it flows into the gills, where it becomes arterial, and 

 returns through other longitudinal channels which run above the pleurovisceral 

 cords. Two transverse channels in the region of the heart (c/. Fig. 51, p. 40) 

 convey the arterial blood into the auricles. 



The two pedal arteries lie laterally and ventrally with regard to the pedal cords ; 

 they probably draw their blood from the aoita and pass it on to the lacunar system 

 of the foot. 



b. Solenogastres. — The heart lies above the hind-gut on the dorsal side of the 

 liericanlium. It does not lie freely in the latter, nor is it suspended by an 

 endothelial band, but simply projects into the pericardium from above, so that only 

 its under surface is I'overed by the pericardial endothelium. The presence of two 

 auricles has not been proved. The rest of the circulatory system is purely lacunar. 

 Specially large blood channels lie in the depths of the principal septa which project 

 into the mid-gut, and bulge these out. Large blood sacs are also occasionally found 

 in folds which project into the pharyngeal cavity from its wall, and there are more 

 or less large sinuses in the folds, which, in Ncomenia and Chcctodcnna, project 

 into the cloaca and may be regarded as gills. In all these parts the intestinal 

 epithelium separating the sinus from the intestine is ciliated, and respiration no 

 doubt takes place. 



2. Gastropoda. 



Relation of the auricles to the ventricle. — The lowest Gastropods, i.e. the 

 Diotocardia among the Prosobranchia, have a heart with two auricles. This is not 

 only the case in the Zeucjobranchia {Fissxirella, Haliotis, etc.), which have two gills, 

 but also in the Azygobranchia {Turbinidce, Trochidce, Neritidce), in which only the 

 left (originally the right) gill has been retained. No branchial vein then enters the 

 smaller (rudiinentary) auricle on the right, the veins having atrophied with the gill. 

 In the Zeugobranchia, the long ventricle lies in a line with the hind-gut, which runs 

 length-wise through it. In the Azygob^-anchia, the ventricle lies transversely with 

 respect to the hind-gut which runs through it, the left auricle lying in front of the 

 ventricle, and the right auricle behind it. The left branchial vein enters the 

 anterior (left) auricle. If we suppose the posterior (right) auricle to have disapi)eared 

 altogether, as is the case in all other Gastropoda, the heart consists of a ventricle 

 and one auricle lying in front of it, wdrich receives the branchial or pulmonary vein 

 from the gill or lung in front of it. 



