200 GASTEROPODA. 



Cyprceidce, and other families, having a spiral shell with a 

 channelled mouth. 



4. PULMONOBRANCHIATA, Sowerby. 



Respiratory apparatus consisting principally of an internal 

 cavity or pouch, formed hy a fold of the mantle, and lined 

 with a network of vessels. Limacidce, Hdicidce, and other 

 lnd and freshwater univalves, besides a few marine kinds, 

 some of which are naked and others provided with shells. 



5. PLEUROBRANCHIATA, Gray. 



GILLS forming a single row, placed on the right side of the 

 body, and covered by the mantle. Bullidce. 



6. NUDIBRANCHIATA, (Nudibranches) Cuvier. 



GILLS exposed, and forming a tuft on the back. Dorididce 

 and most Sea- slugs. 



7. PELLIBRANCHIATA, Alder & Hancock. 



Respiratory apparatus consisting of a net-work of vessels 

 diffused over the outer surface of the mantle. Limapontiidce, 

 and small Sea-slugs of an inferior type. 



8. NUCLEOBRANCHIATA, De Blainville. 



Respiratory apparatus consisting of symmetrical filaments 

 associated with the digestive organs in a nucleus placed on 

 the back. Carinaria and a few other pelagic mollusca of a 

 peculiar kind (Heteropoda), none of which are British. 



In the Prosobranches of Milne-Edwards (which con- 

 stitute the first three orders) the gills are almost always 

 enclosed in a vaulted chamber or cavity, wbicb is placed 

 on the front part of the body; the sexes are separate ; and 

 tbe shell is complete in all stages of growth. In bis 

 Opistbobranches (wbicb constitute tbe fifth and sixth 

 orders) the gills are never enclosed in a special cavity or 



