PHOLAS. 185 
posterior end of the tricuspid membrane ; as soon as it is clear 
of the pylorus, it makes a double and plunges deeply into the 
body, nearly to the foot, through the folds of the liver, and 
then ascends to the dorsal region, to near the poimt from 
whence it commenced the circumvolution ; it then proceeds 
under the peritoneum or membrane enveloping the liver and 
stomach to the pericardium, which it pierces, and passes in a 
straight line, embraced by the ventricle and auricles, to its 
termination in the anal tube. 
The Circulation and the Respiratory Organs. 
The circulation is complete; that is, there is an aortic 
action, and a venous reflux of the blood for aération to the 
pulmonary apparatus. The respiration is effected by a pair 
of very long membranous narrow symmetrical laminz on each 
side of the body, composed of a vascular network, fixed under 
the mantle to the dorsal range, accompanied also on each side 
by a pair of palpi. On leaving the body the four branchi 
without an imtermediate substance run together tapering to 
their termination at about 4 an inch from the extremity of 
the branchial orifice; they are firmly fixed the whole length 
by their bases, from the poimt they leave the body to the 
membrane which separates the anal from the branchial cavity, 
cutting off all communication between the two siphons; con- 
sequently the water must be received and ejected through the 
branchial siphon, or by the pedal orifice, in the bivalves with 
closed mantles, as is the case with the Pholades, and probably 
with other families of the same structure in which the branchial 
and anal tubes are separated their entire length by a divisional 
membrane. 
The discovery of this circumstance is so important as regards 
the disputed pomt, how the branchiz receive the ambient 
element, that I have used every means to test it, by dissection, 
by all the modifications of experiment, and particularly by 
mercurial injections ; this last mode I have used very success- 
fully, as the followmg operations will show. First, I threw 
into the anal siphon a column of mercury that completely 
filled the cavity, and on applyimg further pressure, regurgita- 
