556 



LECTURE XXn. 



ff, to terminate at i, close to the margin of the mantle y, 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 



Faludina 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 : n 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 : / 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 Haliotis. 



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

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

 whelk (Buccinum) a crop-like cascum is developed from the cardiac 

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



* CCCXXI. 



