10 
GASTEROPODA. 
Shell spiral ; adult and larva shell-bearing ; 
larva with deciduous cephalic fins. 
Order IV. Gymnobranchiata. — Gills exposed, 
or contractile into a cavity on the surface of 
the mantle. Adult shell-less; larva shell- 
bearing, with deciduous cephalic fins. 
Order V. Pneumobranchiata. — Respiring free 
air in a closed chamber lined with pulmonic 
vessels. Adult and larva shell-bearing ; larva 
shaped like the parent, without cephalic fins. 
The last order consists almost entirely of terres- 
trial or fluviatile Mollusca, their organisation being 
only adapted for respiring free air ; and there are a 
few fiuviatile species found in the first and second 
order: the rest are all marine, and therefore ex- 
cluded from our consideration at present. 
The operculum of Gasteropodous Mollusca is like 
the shelly valve of those animals ; and the shelly 
valve and the operculum together are homologous to 
the two valves of a conchiferous mollusk. I am 
therefore led to believe that the normal or typical 
form of mollusca is, to be protected by two valves or 
shells. If this theory be correct, the operculum 
should afford an important character for the distinc- 
tion of families and genera. 
As this theory is not generally understood, I may 
add that the operculum of these gasteropodous mol- 
lusca, like the shelly valve of those animals and of 
each of the valves of bivalves, — 
