320 Mr. W. Clark on the Pholadidse. 



would lead us to expect that the medullary masses would be of 

 corresponding importance ; this is not the case, as in Pholas dac- 

 tylus I can only find two inconsiderable ganglia ; the anterior one 

 is the largest, consisting of a white pulpy mass, situated on the 

 centre of the oesophagus just above the buccal aperture ; from it 

 two distinctly visible threads curve anteriorly, the one giving 

 filaments to the right, the other to the left palpum, from whence 

 additional ramifications proceed to the anterior parts of the 

 body, besides supplying the muscles of the foot. The posterior 

 ganglion is situated between the heart and the anus, and is con- 

 nected with the anterior one by two close, parallel, dorsal, very 

 minute longitudinal threads that are seen without difficulty, and 

 cannot well be mistaken for veins; the minor mass furnishes 

 threads to the adductor muscle, and sends to the ovarium and 

 muscles of the belly appropriate filaments. This is all that I 

 have been able to observe of the nervous system, and collect from 

 it, that however insignificant the ganglionic masses may appear, 

 their efi'ects on the muscles prove that the potentiality of their 

 influences is not impaired by the minuteness of the hair-like 

 threads which are the conductors of the subtle fluid that excites 

 their action. 



The Digestive Organs. 



We commence with the mouth, situate immediately above the 

 connecting labium of the palpi ; it is rather a large transverse 

 orifice, and leads directly into the simple oesophagus, which pro- 

 ceeds with a portion of the liver on each side of it through the 

 anterior part of the dorsal range into a small oval stomach, the 

 base of which is enveloped by the light green granular liver 

 which pours the bile into its ridged coat by several ducts ; its 

 cavity is almost filled up with a folded plate, which I call the 

 gizzard or stomachal attritor, and authors the tricuspid mem- 

 brane, which is erroneously, as I think, considered by some ma- 

 lacologists an agent to regulate the entrance of the bile from the 

 liver. I think this idea cannot be sustained, as besides the stomach 

 being provided with bile ducts, one of the axes of the tricuspid 

 corneous plate is fixed at the cardiac orifice of the stomach, and 

 receives the animalculse as they descend the oesophagus ; and after 

 trituration by the gizzard, which is worked by the elastic hyaline 

 stylet, they pass through the other axis of the tricuspid mem- 

 brane, which is inserted in the pyloric orifice into the intestine. 

 I have preparations showing the gizzard in the stomach with its 

 posterior end united to the intestine, and attached by the middle 

 to the hyaline stylet. 



This singular organ, so well known to exist, I believe, in all 

 bivalves, has caused some difference of opinion as to its use ; but 



