BY K. M. JOHNSTON, F.L.S. 



73 



Class GASTEROPODA, (vii.-lxvii.) 

 Head distinct, usually furnished with, eyes and tentacles; 

 body mostly protected by a spiral or conical univalve 

 shell ; lower surface of animal developing a thickened, 

 expanded, creeping disc or foot. 



Sexes 



Sub-Class PROSOBEANCHIATA. 

 separate in different individuals. 



(vii.-li.) 



Mostly marine 

 animals, provided with a shell and generally an 

 operculum, embraces at least all operculated mol- 

 luscs. Animals breathe by gills or branchice. 



Sub-Class OPISTHOBRANCHIATA. (lii.-lvii.) 

 Marine slugs breathing by arborescent or fasciculated branchice, 

 more or less exposed on back and sides posteriorly ; a 

 large division shell less, another possessing a spiral, 

 conical or lamellar shell partly concealing branchiae and 

 itself more or less concealed by mantle lobes. Sexes 

 united. 



Sub-Class PTJLMONATA. (lviii.-lxvii.) 



Mostly terrestrial (land shells), a portion being fluviatile (fresh 

 water), usually provided with a shell without operculum, 

 breathing air by the simplest form of lung, a pouch with 

 external opening, lined with a net work of respiratory 

 vessels. Sexes united in the same individual. 



Class SCAPHOPODA. (Ixviii.) 

 Shell or hollow cylinder, straight or curved without spire, 

 open at both ends. Head rudimentary ; foot vermiform, 

 lobulate ; nervous system simplified, resembling that 

 of the pelecypods or lamellibranchs. 



Class PBLECTPODA. (lxix.-ci.) 

 (Lamellibranchiata Conchifera, Bivalves.) 

 ■No head nor eyes. Animals breathing by lamella?, two on 

 each side, mostly disecious head with two chambers ; 



