CLASSIFICATION 25 
Order I.: OpPISTHOBRANCHIA, 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 
(Aplysiidz), 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.: PutmMonaTa, 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 XIII, 
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 XIII., 
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. 
Cxiass III.: The SCAPHOPODA, or SOLENO- 
CONCHA (Plate XV., Figs. 1 and 2), of which the 
