THE MOLLUSCA 9 
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 
4 ne C f Noe A E 
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f 
Fic. 3. 
Diagram to show the relations of the heart in the Mollusca. 4, part of the dorsal vascular 
trunk and transverse trunks of a Chaetopod worm; B, 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); ai, arteria abdominalis ; v, ventricle. The 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 Molluses 
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 , 
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 
is much reduced, or may be aborted, 
as is the case in the majority of Gas- 
tropods (Fig. 3, E). 
Primitively a single, morphologically 
Vil 
Fic. 4. 
Heart of Chiton pellis-serpentis, dor- 
sal aspect. I, left anterior auriculo- 
ventricular communication; II, ven- 
tricle; III, right anterior afferent 
vessel; IV, V, VI, right posterior 
afferent vessels; VII, left auricle; 
VIII, left posterior auriculo-ventricu- 
lar communication. 
anterior aorta is given off from the ventricle, and this condition 
persists in the Amphineura and in the archaic Lamellibranchia. 
