266 DR. A. RATTRAY ON THE ANATOMY, PHYSIOLOGY, 
A heart (Plate XLIII. figs. 4, 6, x), consisting of an auricle and ventricle, is distinctly 
visible in Firola, in front of the branchial nucleus, and usually in close proximity to the 
ciliated breathing-apparatus when present. 
The ventricle (fig. 6, 1) is oval. From its upper end springs the aorta ; and below, it 
communicates with the auricle. Its walls consist of a thin but clearly defined mem- 
brane, strengthened by a mesh of striated muscular fibres, first pointed out by Huxley, 
by the contraction and expansion of which the blood receives an onward impulse 
through the aorta. The auricle (fig. 6, x), situated below and separated from the 
ventricle by a slight constriction, is much less perfect, and consists in front of a 
museular network like that of the ventricle, only less complete, connected below with 
the ciliated branchial tube, and above with the ventricle, the muscular fibres of the 
two occasionally intermingling. The cavity, of which this forms the anterior wall, 
is large and has more the character of a sinus than a well-defined auricle. The contraction 
of this muscular web necessarily propels a certain portion of the contained blood into the 
ventricle, and, at the same time, draws an equivalent of non-oxygenated fluid from the 
periintestinal cavity or venous system, part of which comes into contact with, and is 
aerated by the ciliated branchial apparatus or branchise when present. The circulation 
thus kept up is languid ; and the function of the singular heart is doubtless not so much 
to keep the blood in a continuous circuit as in perpetual slow motion and constantly 
changing, so as to present a fresh supply to the aerating medium, whether that consists of 
a branchial apparatus, a subspiral band, or merely the venous sinuses of the body generally, 
one or all combined. A more rapid current is not required. No complete double circu- 
lation is needed for the aeration of the blood, nor has all of that fluid to pass quickly and 
repeatedly through the oxidating medium for renewal, as the heart supplies enough 
motive power to keep the blood moving sufficiently fast for all physiological requirements. _ 
The aorta (Plate XLIII. figs. 1 & 4, D), somewhat dilated at its origin, springs from the 
upper part of the ventricle, gives a branch backward to the viscera, and, without diminish- 
ing much, if at all, in its course, runs up the centre of the body near to the cesophagus, but 
lying, like it, close to the ganglia and their intercommunicating cords. Near the pedal 
ganglion it sends a vessel to the swimming-fin (fig, 1, D), then courses on towards 
the buccal mass, where it becomes lost, and doubtless ends, like the pedal artery, in the 
return circulation. With the above exception, the aorta gives off no branches in its entire 
length; but, from its exceeding thinness and transparency, it is apt to be overlooked. It 
has usually a constricted or beaded appearance (fig. 4), from irregular contractions of its 
coat, which seems to consist of a simple membrane, in which no special structure or 
muscular fibres can be detected. According to Huxley there are no special veins in the 
return circulation, and the blood returns to the heart through the periintestinal cavity ; 
and certainly strong evidence in support of this lies in the fact that no tissue can be 
detected between the muscular coat and contained viscera, which appear to float free in 
the blood of this rudimentary venous system—and among them, curiously enough, the 
vessels of the efferent circulation, viz. the aorta, pedal and branchial arteries, blood-vessels 
bathed on both sides in blood, which thus convey the aerated fluid direct and unmixed 
from the breathing-apparatus to the active organs which most require it, e. g. the liver, 
