556 LECTURE xxn. 



g, to terminate at i, close to the margin of the'mantle/, which forms 

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

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



204 



Paludina vivipara. 



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 : 9i is the 

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

 and ventricle : h is the long and wide oviduct, which performs the 

 office of the uterus in this ovoviviparous species : I is the duct of a 

 mucous or renal organ attached to the walls of the branchial cavity.* 



The disposition of the viscera of other Gastropods 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. 



The oesophagus is comparatively short in Helix {Jig. 207. e.), and 

 shorter in Thetis and HoJiotis, 



In a few Gastropods, the pond-snails {LyynncBa^ Planorbis), for 

 example, the oesophagus presents a small ingluvial dilatation : in the 

 whelk {Buccinum) a crop-like caecum is developed from the cardiac 

 end of the stomach. The gastric crop is wider in Aplysia, in which the 



* CCCXXI. 



