Mr. W. Clark on the Pholadidie. 333 



rectum, opens into the tubular mautellar canal at some distance 

 from the anal siphon ; the whole of the circumvolution is about 

 5 inches, far exceeding that of the Pholades. The valvular dorsal 

 flap I have before mentioned covers the aperture under it, assisted 

 by a fine membrane perforated to correspond with the oval aper- 

 ture, which appears to be in aid of the external valve for pre- 

 venting the ingress or egress of water, except at the minute per- 

 foration, in unison with the true mouth. I can conceive no other 

 use for this valve than to admit water to the mouth, oesophagus 

 and stomach when the foot is engaged hi excavation, and in con- 

 sequence perhaps the anterior gape is hermetically closed. 



It would appear that the animal swallows the excavated wood, 

 and does not eject it by currents of water. I infer this, as not 

 only the stomach but the intestine is always filled with a pulp, 

 which under the microscope has the aspect of ligneous debris. 

 The Patella operate in like manner. 



The circulation is venous, arterial and branchial, and conse- 

 quently complete. The respiratory apparatus has been strangely 

 misunderstood ; it has been described to consist of four fleshy 

 cords, portions of which Sir Everard Home pronounced to be the 

 testes, and others the ovaria ; these views are erroneous. But 

 we will first mention the heart and auricles, which are placed at 

 the base of the ovarium in the peritoneal cavity within the man- 

 tle, but in a distinct pericardium ; the heart is an elongated, very 

 pale bluish white opake ventricle, accompanied by two symme- 

 trical fusiform slender auricles that are also opake, somewhat 

 posterior to it, which appear to pour the aerated blood into it by 

 lateral valvular ducts ; on opening the ventricle its walls did not 

 exhibit any particular muscularity ; we were not successful in de- 

 tecting the valves of the aui-icles. There is at the posterior part 

 of the auricles a white, suboval, subglobular, fine granular mass, 

 touching and partly surrounding them ; we are unable to state 

 its nature ; it is not part of the ovarium, which terminates before 

 the pericardium commences, and in such a situation it cannot be 

 the organ to animalize the ova : I am inclined to consider it a 

 gland that distils a liquor for the use of the heart and auricles. 



At the base of these organs the four cords that have created 

 such difference of opinion as to their uses come into view, but 

 they do not appear to be either the branchiae, arteries, veins, 

 testes or ovaria; still they have a sort of connection with the 

 branchiae ; the two longer and larger brown lines have their ori- 

 gin on each side the hemispherical valves, and proceed, attached 

 to each latero-dorsal range of the mantle, to the posterior siphons ; 

 they appear to be composed of red brown granular points ; within 

 these two lines, but not until the branchiee commence, two others 

 of smaller size and nearly similar composition run parallel, and 



