Classes 
PELECYPODA 
(Bilaterally symmetrical, 
headless, bivalved mol- 
lusks. Mussels, cockles, 
oysters, etc.) 
CEPHALOPODA 
‘Mollusks of high organi- 
zation, having a definitely 
formed head surrounded 
by tentacles ; shell, when 
present, usually internal; 
shell, external in nautili. 
Cuttlefishes, squids, oc- 
topi, and nautili) 
Subclasses 
Tetrabranchiata 
Dibranchiata 
MOLLUSKS 
Orders 
PROTOBRANCHIATA 
(Gills form a single pair of plume- 
like organs, each with two rows of 
flattened gill-filaments) 
FILIBRANCHIATA 
(Two pairs of plate-like gills formed 
of V-shaped filaments) 
PSEUDOLAMELLIBRANCHIATA 
(Gills plaited so as to form vertical 
folds) 
EULAMELLIBRANCHIATA 
(Firm, basket-work gills, the jila- 
ments united by vascular connec- 
tions) 
OCTOPODA 
DECAPODA 
