387 



GASTEROPODA PROSOBRANCHIATA. 



The great majority of our marine univalve shells are 

 the body-coverings of sea-snails which have the lower 

 part of their bodies formed into a thickened creeping 

 disk or foot, and their organs of respiration lodged, in 

 the shape of comblike gills, in a vaulted pallial cavity; 

 except a few genera, which have these organs lining, as it 

 were, the inner margin of the mantle. The shell is partly 

 devoted to the protection of the gills and partly to that 

 of other viscera. The heart is in most of them placed 

 behind the gills. The sexes are distinct. In common 

 with all other gasteropods which breathe by branchiae 

 they affect in their larva state a form reminding us of 

 the Pteropoda, being then furnished with broad ciliated 

 fins springing from the sides of the head. Their shells, 

 except in a few instances, are spiral, and the larvaj 

 appear to be always furnished with spiral shells, what- 

 ever shape these important appendages may eventually 

 assume. 



The term Pro&obrancliiata was proposed for these mol- 

 lusks by Milne Edwards,* who constitutes of them his 

 second order of Branchiferous Gasteropods. Availing 

 ourselves of the views promulgated by that eminent zoolo- 

 gist, respecting the classification of the Gasteropoda — 



* Sur la Classification Naturelle des Molliisriues Gasteropodes, Annale 

 des Sciences Naturellcs for February, l84o. 



