MOLLUSCA. 207 
The creeping tube, which unites the individuals of a 
group, is the channel for two separate currents of blood, an 
upward and a downward one, that are flowing at one and 
the same time, and that send off each branch to every pe- 
duncle : the blood thus passes into the animal by one cur- 
rent, while another carries it back. One of these canals 
communicates at the termination of the peduncle with the 
heart, which is placed, as has been mentioned, near the bot- 
tom of the branchial sac on the left side, and consists of a 
transparent ventricle, or boyau, running forward and a little 
slopping downward, in a channel hollowed to contain it. 
Along the whole length of the boyau a part on one side of 
its axis seems fixed to the channel, the rest free and con« 
tractile. 
When the blood entered the heart from the peduncie, 
contraction began at the middle of the ventricle, impelling 
onward the contents of the fore part ; and the contraction 
of the back part followed in the same direction, so as for the 
whole to have the effect of one pulsation ; the heart was 
then filled again by a flow from the peduncle. The inter- 
vals of the pulse were pretty regular in the same individual, 
but in different ones they varied from two seconds to one 
and a half second. Part of the blood thus impelled formed 
a main upward stream along the front of the branchial or- 
gan, branching off at each of the horizontal passages between 
the rows of spiracles, and at one above them on a line with 
the junction to the mantle on each side. All these again 
united and formed a downward current behind. The hori- 
zontal channels were connected also by the smaller vertical 
passages between the spiracles; the set of the current in 
the latter being upwards for the two lower rows, and down- 
wards for the two upper ones. 
