304 THE CEPHALOPODA 
Dibranchia, have a more complete and perfect circulatory system 
than other Mollusca, the blood being nearly entirely contained in 
true vessels. 
The heart is situated somewhat superficially near the middle of 
the posterior or physiologically ventral surface (Fig. 252, h). It lies 
in a pericardial cavity (Fig. 252, coe), except in the Octopoda, in 
which sub-order this cavity is much reduced (Fig. 278, ca). The 
essential part of the heart is the median ventricle (Fig. 277, ven), the 
lateral and symmetrical auricles being nothing more than simple 
Fic. 270. 
Diagram representing an approximately median sagittal section of a female Nautilus 
pompilius. The parts which are quite black are the cut muscular surfaces of the foot and buccal 
mass. «, the shell; an,anus ; a.b.v, afferent branchial vessel ; b, the nuchal plate ; c, the integu- 
ment covering the visceral hump; cr, crop; d, the mantle flap or skirt in the dorsal region, 
where it rests against the coil of the shell; e, the inferior margin of the mantle-skirt resting 
on the lip of the shell represented by the dotted line; e.b.v, efferent branchial vessel; f, the 
pallial chamber with two of the four gills ; g, the vertically cut median portion of the funnel ; 
gizz, gizzard ; h, the capito-pedal cartilage ; i, the valve of the funnel ; int, intestine; 7, the 
siphuncular pedicle (cut short); m, the hood or dorsal enlargement of the outer lobe of the foot ; 
n, tentacles of the outer lobe; x.c, nerve-collar ; nept, aperture of the kidney ; nz, nidamental 
gland ; oe, oesophagus ; p, tentacles of the inner inferior lobe; g, buccal membrane ; 7, upper 
jaw ; r.e, renal glandular masses on the walls of the afferent branchial veins ; s, lower jaw; t, 
radula; vt, ventricle of the heart ; x, the viscero-pericardial sac. (After Lankester.) 
contractile expansions of the efferent branchial vessels (Fig. 277, aw). 
In Nautilus there are four such auricles, returning blood from the four 
branchiae, but in the Dibranchia there are only two. In general the 
ventricle is slightly asymmetrical, except in Nautilus (Fig. 274, vent), 
in which it is transversely elongated, and in Loligo, in which it is 
elongated antero-posteriorly. The entrances of the auricles into the 
ventricle and origins of the aortae from it are guarded by valves. 
The aortic vessels consist of —(1) a cephalic or principal aorta, which 
runs forward (Fig. 277, a.a) and supplies the whole of the anterior 
part of the body with blood ; (2) a posterior or abdominal aorta, which 
is smaller, especially in the Octopoda, and carries blood to the 
