MOLLUSCA. 425 



The organs of circulation consist merely of veins and ar- 

 teries. The veins which 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 abdomen, 

 divides into two branches, each of which may be considered 

 as a vena cava, conveying the blood to the lateral hearts. 

 Each vena cava, at its origin, is joined by an equally large 

 vessel, which empties its contents in a direction nearly at 

 right angles with the former. These veins arise in the sto- 

 mach, 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 branchiae. 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, 

 jit readily passes through the glands into the bag, and 

 thence into the cavity of the tunic ; and when air is blown 

 into the bag, it likewise penetrates the gland, and passes 

 into the veins. The arteries with which these glands are 

 furnished are comparatively minute. 



It appears probable that these glands separate some 

 principle from the blood, and that this is conveyed away 

 by the ejection of the water from these venous bags into 



