Mr. W. Clark on the Pholadidse. 321 



I think when all the incidents attached to it have been men- 

 tioned, they, in conjunction with the position in the stomach of 

 the tricuspid organ, can lead to no other conclusion than that the 

 apparatus is a gizzard worked by the foot and elastic stylet to 

 comminute the food, and is analogous to the gizzard in many of 

 the Gasteropoda. The stylet is for the basal half cylindrical, and 

 tapers from thence to the stomach, where it makes a loop, and is 

 fixed by a filamentary muscle to the gizzard or tricuspid mem- 

 brane ; its colour is hyaline milk-white, and in certain lights 

 reflects the metallic hues ; the working point of support is the 

 centre of the basal part of the foot, through the pedicle of which 

 it proceeds obliquely to the stomach, guarded by a sheath which 

 appears to secrete a lubricating fluid, probably having its source 

 from the liver, through the centre of which it passes to its junc- 

 tion with the corneous attritor ; it is eminently elastic, formed of 

 a suite of circular lines ; it is impervious. I have submitted it to 

 every sectional form, but the only departure from homogeneity 

 are the fine circular elastic fibres ; in the species we now describe 

 it is fixed by a short muscle to the bottom of the foot ; in P.parva 

 it appears to rest, free. At one time I thought the stylet might 

 be the vehicle of a solvent fluid from the stomach, but its imper- 

 meability negatives this idea ; and if there is a connection with 

 the foot from the stomach, it must be by the sides of the walls 

 of its sheath ; in that case a solvent would neutralize the lubricity 

 so necessary to its action, as a spring for the gizzard ; besides, the 

 most careful examination of the external and internal surfaces of 

 the foot shows no connection between them, or orifice for the 

 issue of a solvent. No adjuvant powers of sight have enabled me, 

 in this species, to discover the pore which is said to admit water 

 to the foot of many of the bivalves, or to expel it if received from 

 the stomach. 



I now return to the intestine, which we left united to the pos- 

 terior 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 livei", 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 iss 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 mem- 



