CLASSIFICATION 25 



Order I. : Opisthobranchia, so called because the 

 heart is in front of the gills. 



Sub-Order 1 : Tectibranchia (Plate XII., Figs. 

 6-19), or those having shells, which are mostly 

 fragile or rudimentary, and often concealed in the 

 folds of the mantle. To this belong the Sea-Hares 

 (Aplysiidse), the Bullas, etc. 



Sub-Order 2: Nudibranchia (Plate XII., Figs. 

 20-28), or the shell-less Sea-Slugs, are externally 

 symmetrical animals, without true gills, respiration 

 being effected by secondary gills, or by the general 

 surface, aided in some cases by the " cerata," which 

 are appendages of the dorsal integuments. 



Order II. : Pulmonata, or the majority of land 

 and freshwater shells, in which the true gill has 

 disappeared, and its function is taken up by the wall 

 of the mantle cavity. 



Sub-Order 1 : Basommatophora (Plate XIIL, 

 Figs. 1-10), or those having eyes at the bases of the 

 tentacles, embraces the freshwater pulmonates and 

 a few land shells. 



Sub-Order 2 : Stylommatophora (Plate XIIL, 

 Figs. 11-35, and Plate XIV.; Radula, Plate III., 

 Figs. 21 and 22), or those having the eyes at the 

 end of the " horns," takes in nearly all the land 

 shells. 



Class III. : The SCAPHOPODA, or SOLENO- 

 CONCHA (Plate XV., Figs. 1 and 2), of which the 



