292 Introduction to A n imal Morphology. 



in a thin-walled pericardium,* in front and above the 

 genital gland ; Jt consists of one (usually) or twof 

 auricles, always [directed towards the breathing 

 organs, and one round or pyriform ventricle, in some 

 genera pierced 'by the intestine. + There are two, 

 rarely one, auriculo-ventricular, and two aortic valves. 

 The ventricular muscle is red,§ and its fibres pass in 

 all directions,'crossing its cavity as trabeculae. The 

 blood may be bluish, and richly corpusculated (Pul- 

 monata) transparent (Heteropoda), rarely red (Planor- 

 bis), usually whitish opaline, and containing copper 

 in some Pulmonates [Harless and Wickc). 



The auricle is larger than the ventricle in Hetero- 

 poda, but is represented only in Abranchiates by 

 muscular threads surrounding the inlet of the ven- 

 tricle. 



The ventricle gives off an aorta, which soon divides into 

 cephalic and abdominal branches ; the aorta, springing from 

 a bulbus in Heteropoda, divides into corporal and visceral 

 trunks corresponding to these, and the former divides into 

 caudal and pinnal branches for the meta- and epipodium ; 

 and the caudal branch in Carinaria divides into two lateral 

 branches and a medial penial artery in the male. These 

 vessels in Pulmonates are white, being full of calcareous con- 

 cretions. Where the rectum pierces the heart there are two 

 aortae (as well as two auricles), which unite, and then divide 

 into the two primary branches. The cephalic aorta in Proso- 

 branchs and Pulmonates gives off cerebral and pedal arteries. 



* To which it is tied by fibrils in Prosobranchs ; it is free in front of 

 the visceral nucleus in Heteropoda. 



t Two in Janus, Haliotis, Fissurella, Emarginula, Chiton, &c. 



J Chiton, Nerita, Neritina, Turbo, Emarginula, Fissurella, Parmo- 

 phorus. 



§ Especially in Pulmonates ; its fibres are branched and polynuclear in 

 Heteropoda. 



