126 MOLLUSCA. 
hepatic ducts, which proceed to the extremity of the spiral 
stomach, where, by a common orifice, they empty the 
orange-coloured bile which they contain. 
The organs of circulation consist merely of veins and ar- 
teries. ‘Theveinswhich have their origin in the feet, mouth, 
and annular cartilage, coalesce, and form two branches, 
which afterwards unite into a common trunk. _ This vessel, 
after descending through part of the viscera into the abdo- 
men, divides into two branches, each of which may be con- 
sidered as a vena cava, conveying the blood to the lateral 
hearts. Each vena cava, at its origin, is joined by an equal- 
ly large vessel, which empties its contents in a direction 
nearly at right angles with the former. These veins arise 
in the stomach, intestines, liver, and organs of generation. 
The vena cava receives a second large vessel, nearly in the 
same direction as the first, which has its origin in the tunic 
and the supports of the branchie. From the size of the 
vena cava, in consequence of the union of these two branches, 
and the appearance of muscular ridges on its inner surface, 
it has been compared by some to an auricle. 
On each side, in the common cavity of the tunic, and 
near the gills, an aperture may be observed, the entrance 
to a bag or cavity. Each cavity is traversed by the vena 
cava of that side, and in its passage exhibits a curious con- 
formation. The surface of the vein is covered with spongy, 
glandular bodies of different shapes. These, upon being 
pressed, pour out an opake, yellow, mucous fluid. Within, 
these glands communicate by very wide ducts with the ca- 
vity of the vein. Indeed, when air is blown into the vein, 
it readily passes through the glands into the bag, and thence 
into the cavity of the tunic; and when air is blown into 
