32 MR. T. H. HUXLEY ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA. 
The Circulatory System (figs. 1, 2, 3, 6).— This consists of a very perfect heart 
with an aorta and its branches, but there is no trace of any venous system. 
The heart lies anterior to and parallel with the rectum ; its axis therefore is neailv 
at right angles with that of the body. It possesses two chambers, an auricle (w) and 
a ventricle {v). The auricle is large and somewhat elongated, extending above into 
the elevation which carries the subspiral ciliated band. Its vrall is formed by 
branched and interlacing muscular fibres, which are attached partly to the parietes, 
partly to the walls of a contractile sac (c), to be mentioned presently. 
The ventricle is almost globular, and has thicker walls, in which the separate 
muscular fibres are not distinguishable. The lips of the auriculowentricular aperture 
are prolonged on the ventricular side into two valvular folds. Below, the ventricle 
terminates in a wide aorta, which immediately gives off a large branch backwards 
to the hepatic and generative organs; then becoming much convoluted, it runs 
forwards along the intestine and stomach, passing between the latter and the pedal 
ganglia (fig. 6), and finally terminating, without much alteration in its diam.eter, m 
the buccal mass. 
As it passes over the pedal ganglia it gives off a considerable branch, the pedal 
artery,” downwards to the foot ; and this pedal artery, just before it enters the foot, 
gives off a long and delicate metapodal” branch {w'), which passes backwards, 
parallel to the aorta, and finally terminates in the inetapodium, figs. 2 and 3. 
The mode of ending of the pedal artery is very remarkable, and physiologically 
speaking, almost unique (fig. 6). Having entered the foot, it ends suddenly, without 
narrowing, in a truncated open extremity {y’). In the living animal this open end 
possesses the power of contracting to a very great extent, so as almost to become 
closed; and its condition must necessarily exercise a very considerable influence 
upon the direction and rapidity of the animal s circulation. 
FiroMdes* then affords the most complete ocular demonstration of the truth of 
M.-Milne Edwards’s views with regard to the nature of the circulation in the 
Mollusca, that can possibly be desired. The perfect transparency of the creature 
allows the corpuscles of its blood to be seen floating in the visceral cavity between 
the intestine and the parietes, and drifting more or less rapidly backwards to the 
heart. Having reached the wall of the auricle, they make their way through its 
meshes as they best may, sometimes getting entangled therein, if the force of the 
heart has become feeble. From the auricle they may be followed to the ventricle, 
and from the ventricle into the aorta, whence they pass, some forwards, to the buccal 
mass, in which the aorta ends, and through whose tissues it pours them ; some 
downwards, to pour out of the widely open end of the pedal artery, flooding the 
tissues of the propodium ; and a small proportion passes directly backwards to the 
visceral mass and to the metapodium. 
* A similar condition of the circulatory system has been observed by Nordmann, Quateefages, Van 
Benlden and Allmann, in various Nudibranchiata, though perhaps not quite so distinctly. 
