MOLLUSC A. 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 peduncle, 

 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. 



