398 PHYLUM MOLLUSCA, 
through the ventricle of the heart, and ends above the 
posterior adductor at the exhalant orifice. 
Vascular system.—The heart lies in the middle line on 
the dorsal surface, within a portion of the body cavity called 
the pericardium, and consists of a muscular ventricle which 
has grown round the gut and drives blood to the body, 
and of two transparent auricles—one on each side of the 
ventricle—which receive blood returning from the gills and 
mantle. In bivalves the heart-beats average about twenty 
per minute, much less than in Gasteropods. The colour- 
less blood passes from the ventricle by an anterior and a 
posterior artery ; flows into ill-defined channels ; is collected 
in a “vena cava” beneath the floor of the pericardium ; 
passes thence through the kidneys, where it loses nitrogenous 
waste, to the gills, where it loses carbonic acid and gains 
oxygen; and returns finally by the auricles to the ventricle. 
The blood from the mantle, however, returns directly to the 
auricles without passing through kidneys or gills, but 
probably freed from its waste none the less. The so-called 
“organ of Keber” consists of “ pericardial glands” on the 
epithelium of the pericardial cavity. They seem to be 
connected with excretion. Many of the cells lining the 
blood channels secrete glycogen, the principal product of 
the Vertebrate liver. 
Respiratory system.—Lying between the mantle flaps 
and the foot there are on each side two large gill-plates, 
whence the title Lamellibranch. They are richly ciliated ; 
their internal structure is like complex trellis-work ; their 
cavities communicate with the supra-branchial chamber. 
As in many other Molluscs, the gills or ctenidia are not 
merely surfaces on which blood is purified by the washing 
water-currents (a respiratory function), but some of their 
many cilia waft food-particles to the mouth (a nutritive 
function), and in the females the outer gill-plate shelters 
and nourishes the young larve (a reproductive function). 
The water may pass ¢hrough the gills to the supra-branchial 
chamber and thence out again, or over the gills to the 
mouth, and thence into the supra-branchial chamber. It is 
likely that the mantle has no small share in the respiration. 
In many cases, e.g. Lutraria elliptica, the posterior end of 
the mantle gives origin to a contractile respiratory siphon, a 
