THE MOLLUSC A 



vena cava of Nautilus, in the auricle of Patella (Fig. 80), and in the 

 afferent branchial vein of Aplysia. 



The central and pulsatile portion of the circulatory apparatus 

 is well developed, except in the Entoconchidae. It is situated 



Fio. 3. 



Diagram to show the relations of the heart in the Molluscs. A, part of the dorsal vascular 

 trunk and transverse trunks of a Chaetopod worm ; If, ventricle and auricles of Nautilus ; C, 

 of a Lamellibranch, of Chiton, or of Loligo ; D, of Octopus ; E, of a Gastropod, a, auricle ; a.c, 

 arteria cephalica (aorta) ; a/, arteria abdominalis ; v, ventricle. Tho arrows show the direc- 

 tion of the blood -current. (From Lankester, after Gegenbaur.) 



on the dorsal side in the pericardium, except in Anomia and the 

 Octopods, and originally at the posterior end of the animal. In no 

 case is the pericardium a blood sinus ; it is a portion of the coelom, 

 without communication with the circulatory system, as is shown by 

 the absence of red corpuscles in the pericardium of such Molluscs 

 as have red blood (Penrose and Ray Lankester). 



The heart is entirely arterial, and comprises, firstly, a median 

 ventricle, with muscular walls and internal fleshy columns, the 

 fibres of which give the appearance of M 



striation : being spongy in texture, this 

 organ has no intrinsic nutritive vessels. 

 Secondly, two (in Nautilus four) paired 

 auricles, disposed symmetrically on 

 either side of the ventricle. The 

 auriculo- ventricular openings are simple, 

 except in the Polyplacophora, where 

 they are frequently multiple (Fig. 4). 

 Each opening is provided with a valve 

 opening towards the cavity of the 

 ventricle and preventing the reflux of 

 blood into the auricle. Frequently one 

 member of the single pair of auricles sal ? 8 .i )ec t. i, left anterior anricuio- 



& ^ ventricular communication ; II, ven- 



18 much reduced, Or may be aborted, tricle; III, right anterior afferent 



as is the case in the majority of Gas- Sent vessels -, V vnrieft ^urtcio' 



trrmork /Fio- 3 TT\ VIII, left posterior auriculo- ventricu- 



S ^lg. 6, &). lav communication. 



Primitively a single, morphologically 



anterior aorta is given off from the ventricle, and this condition 

 persists in the Amphineura and in the archaic Lamellibranchia. 



FlO. 4. 

 Heart of Chiton pellis-serpentis, dor- 



