. 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 point 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 aeration to the 

 pulmonary apparatus. The respiration is effected by a pair 

 of very long membranous narrow symmetrical laminae 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 branchiae 

 without an intermediate substance run together tapering to 

 their termination at about \ an inch from the extremity of 

 the branchial orifice ; they are firmly fixed the whole length 

 by their bases, from the point 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 point, how the branchiae 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 following operations will show. First, I threw 

 into the anal siphon a column of mercury that completely 

 filled the cavity, and on applying further pressure, regurgita- 



