218 PHOLADID^E. 



adductor muscle of bivalves, which is here, as in Pholas, 

 wanting, to the remarks on which we particularly refer. 



I close these observations by stating, that the masses of 

 the foot, medial adductor, and posterior sphincter supply their 

 respective parts of the body with the minor muscular threads. 



It is time to inquire about the nervous agencies that 

 stimulate the action of these powerful muscular organs, and 

 we find their volume, as in Pholas, to be apparently not in 

 accordance with them. There are just above the mouth two 

 minute ganglia so nearly confluent that they may be con- 

 sidered as one ; from them, two very slender threads descend 

 to the roof of the anal aperture, distributing numerous rami- 

 fications to the proper stomach and foot, whilst the main cords 

 pass into the base of that portion of the tubular mantle which 

 contains the liver, ovarium and pericardium, in a distinct 

 independent membrane that may be called a peritoneum, and 

 in their passage under these organs they furnish them with 

 filaments, and then piercing the fundus of the peritoneum 

 enter the pericardium, and form a junction with a second 

 larger ganglion that is fixed in that cavity in some measure 

 enveloped by the heart and auricles, and is only visible when 

 the pericardium is cleared of them: this mass supplies the 

 terminal part of the ovarium, the entire branchiae, and all the 

 posterior parts of the body with nervous threads. 



The digestive organs next present themselves. Authors 

 have said there are two distinct stomachs ; this is not so : 

 they have mistaken the peritoneal cavity containing the liver, 

 ovarium and pericardium, for one : the true and only stomach 

 is within the hemispherical valves, in immediate contact with 

 the greenish-brown liver that pours the bile into it from above ; 

 it is very small ; the walls are simple ; and the elastic stylet 

 and gizzard, which some naturalists denominate the tricuspid 

 membrane, work within it as a gizzard and attritor. 



I have carefully dissected the apparatus of the present 

 species, and have it on a card in a united state, showing the 

 hard horny parts of the rubbing portion. Some authors say 

 that this machine is not to be found in all bivalves. Which are 

 without it ? I am inclined to think that it is absent in none. 



