372 



MOLLUSCA. 



The vascular system presents numerous and important modifica- 

 tions. The heart is enclosed in a special pericardium, and is usually 

 placed on one side of the middle line, more or less anteriorly, near 

 the respiratory organs. It usually consists of a conical ventricle 



(Fig. 280) which gives off the 

 aorta, and of an auricle which is 

 turned towards the respiratory 

 organs, and into which the blood 

 passes by the respiratory veins. 

 In some Gastropods (Rhipidoglossa, 

 except Helicina) there are two 

 auricles (the right one being usually 

 the smaller), and the ventricle is 

 pierced by the rectum (Fig. 297 b, c}. 

 In the Chitonidae alone is the heart 

 not only symmetrical (with two 

 symmetrical auricles), but placed 

 at the hind end of the animal. 



When there is only one auricle it 

 is the left, though in the majority 

 of the Streptoneura and of the 

 Pulmonata, and in Actaeon, Lima- 

 cina, Clio virgula and C. adcula 

 it lies in front of the ventricle, 

 and in most Opisthobranchs, in 

 Testacellidae, Onchidiidae, Firo- 

 lidae, and some Calyptraeidae it 

 is behind the ventricle. 



The aorta, which is given off 

 from the end of the ventricle 

 opposite to the auricle, or from 

 the hind end in the Rhipidoglossa 

 with two auricles, usually divides 

 into two arteries, of which one 

 passes forward and branches to the 

 head and foot, while the other 

 passes dorsalwards to the viscera. 

 There is a bulb, extra- or intra- 

 pericardial, on the root of the 

 aorta in some forms (Patella, etc.). 

 The arteries terminate by opening 



FIG. 295. Enlarged dorsal view of the 

 alimentary canal of Murex (from Pel- 

 seneer, after Haller). I duct of gland 

 of Leiblein ; II oesophagus ; /// duct of 

 liver; IV liver; V stomach; VI anal 

 gland ; VII anus ; VIII gland of Leib- 

 lein ; IX crop or proventriculus ; X 

 salivary gland ; XI radula ; XII mouth. 



