OYSTERS, ETC. 53 
two parts, either double or single, and the orifices are 
surrounded by tentacles. 
Circulation.—The blood is purified (aérated) and food 
obtained by the same action. Water is drawn into the 
siphon opening farthest from the valve (Fig. 56, 2), and, 
wafted along by the cilia, is thus brought in contact with 
the gills. The food-particles in the water are carried along 
to the toothless mouth (Fig. 55, 0), that, guarded by two 
pairs of sense organs (palpi), is placed at the end of the 
shell opposite the siphon. They then pass into the stom- 
ach and intestine that winds about and passing through 
Z ex, tn, 
Fic. 56.—Bivalve with siphon, showing the foot. 7, ligament; F, foot; m, 
mantle; s, siphon; ex., excurrent orifice ; 2#., incurrent orifice. Except 
the siphon and the large foot, the arrangement of organs is similar to 
that of the oyster. 
the heart finally connects with the siphon nearest the valve 
(Fig. 56, ex.), out of which all resectementa passes. Extend- 
ing through the body is a glass-like rod, that is possibly a 
brace or support. The heart lies near the hinge (Fig. 55, 
h), and is composed of one ventricle and one auricle in 
the oyster. In other species the heart is three-chambered, 
or there may be two hearts of two chambers each. Arte- 
ries lead from the ventricle over the body, and veins carry 
blood to the gills g, where it is purified by the air in the 
water—then passing back to the auricle. The nervous 
system consists of three pairs of ganglia—cerebral, pedal, 
and a pair that send nerves to the internal organs, gills, 
