GASTEROPODA. 



299 



retracted state. The oesophagus 5' is long and slightly convoluted ; q' 



is the last bend which the tube makes before expanding into the 



121 



Paludina vivipara. 



Stomach, r : s, / show the folds of the intestine in the substance of the 

 liver and ovary ; it penetrates the branchial chamber at s^^, in which 

 the rectum, ty is seen passing along the base of the pectinated gills, g, 

 to terminate at i, close to the margin of the mantle f^ whicli forms 

 the branchial aperture. The letter a indicates the foot in its state of 

 contraction, when its inferior or ambulatory surface is bent trans- 

 versely upon itself : b shows the operculum attached to the posterior 

 part of the foot : c are the tentacula, with the attached ocelli : d is 

 the small siphon which projects below the right tentacle : n is the 

 heart, which consists, as in almost all Gasteropods, of a single auricle 

 and ventricle : h is the long and wide oviduct, w hich performs the 

 office of the uterus in this ovoviviparous species of Gasteropod : lis the 

 duct of a mucous or renal organ attached to the walls of the branchial 

 cavity. 



The disposition of the viscera of other Gasteropods offers few im- 

 portant deviations from that in the Paludina vivipara ; but some of 

 the peculiarities in the structure of certain organs deserve special 

 mention. 



In a few Gasteropods, the whelk, for example, the oesophagus pre- 

 sents a small ingluvial dilatation : the crop is wider in the AplysicB, 

 in which the coats of the second stomach, or gizzard {Jig. 122.), are 



