58 Descriptions of Preparations. 



19. Fresh- WATER Mussel {Anodonta Cygnea), 



Dissected so as to show its digestive tract, with a neural flexure formed by it at 

 its commencement. 



The mantle and the lamellae of the gills have been removed from 

 the animaFs right side_, together with so much of the body-walls 

 as it was necessary to remove so as to expose the coils of intestine, 

 and the stomachy in situ, and in connection with the liver and 

 reproductive gland. The passage from the mouth to the stomach 

 is similarly laid open^ and is seen to be lined with a smooth mem- 

 brane, lyiiig" i^pon the anterior adductor and the anterior retractor 

 muscles, and thrown by their contraction into corrugations. The 

 ingestion of alimentary matters depends, as no prehensile apparatus 

 exists in the Lamellibranchiata, upon the currents of water which 

 find entrance into the digestive tract, and have their passage along 

 it greatly promoted by the readiness with which the circumjacent 

 tissues become charged with, and also set free from the transuding 

 fluid ; by the alternate opening and closing of the valves ; and, 

 finally, by the ciliary movement of the microscopic epithelium 

 which clothes the interior of the digestive tube as well as the neural 

 surfaces of the mantle, gills, and labial tentacles. The bilamellate 

 oral tentacle of the left side is seen to become continuous with the 

 lining membrane of the mouth, to which, by its junction with its 

 fellow of the opposite side, removed in this preparation, it furnished 

 lips. Further down, the liver is seen supporting the lining mem- 

 brane of the stomach and sending ducts to open into its cavity. 

 The diverticulum for the 'crystalline style,' an organ somewhat 

 variable, but found in greatest size and constancy after the winter, 

 opens on the right side of the stomach ; whilst the pylorus leads 

 into the intestine from the left in the preparation. A black bristle 

 is passed along the first segment of the intestine which is seen to 

 form, when viewed in connection with the stomach, a curve with its 

 concavity towards the pedal ganglion. This, the primary flexiu-e of 

 the intestine, is thus a ' neuraP flexure ; the concavities, however, 

 of the other loops which the intestine alone describes, look towards 

 the dorsal or ' haemaP surface. Considering the intestine exclu- 

 sively of the stomach, we may speak of it roughly, as describing 

 three concentric curves in intimate connection with the viscera, and 



