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 



a b.v. 



Fio. 270. 



Diagram representing an approximately median sagittal section of a female Xnittilus 

 jtnmpiliu*. The parts which are quite black are the cut muscular surfaces of the foot and buccal 

 mass, o, the shell ; an, anus ; .?>. t>, afferent branchial vessel ; b, the nuchal plate ; c, the integu- 

 ment covering the visceral hump; /, crop; </, 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 ; /, the 

 pjillial chamber with two of the four gills ; g, the vertically cut median portion of the funnel ; 

 yizz, gizzard ; h. the capito-pedal cartilage ; t, the valve of the funnel ; int, intestine ; I, tlie 

 siphuncular pedicle (cut short) ; m, the hood or dorsal enlargement of the outer lobe of the foot ; 

 ?i, tentacles of the outer lobe; n.e, nerve-collar; nept, aperture of the kidney ; ni, nidamental 

 gland ; oe, oesophagus ; p, tentacles of the inner inferior lobe ; tj, buccal membrane ; r, 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 I^ankester.) 



contractile expansions of the efferent branchial vessels (Fig. 277, au). 

 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 Loligi, 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 



