MOLLUSKS 



301 



Classes 



PELECYPODA 



( Bilaterally symmetrical, 

 headless, bivaloed mol- 

 lusks. Mussels, cockles, 

 oysters, etc.) 



CEPHALOPODA 



C Mollusks of high organi- 

 zation, having a definitely 

 formed head surrounded 

 by tentacles ; shell, when 

 present, usually internal; 

 sltell, external in nautili. 

 Cuttlefishes, squids, oc- 

 topi, and nautili) 



Subclasses 

 Tetrabranchiata 



Dibranchiata 



Orders 



PEOTOBEANCHIATA 



(Oillsform 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) 



PSEUDOLAMELLIBKANCHIATA 

 (Qills plaited so as to form vertical 



folds) 



EULAMELLIBRANCHIATA 

 (Firm, basket-work gills, the fila- 

 , ments united by vascular connec 

 \ tions) 



OCTOPODA 

 DECAPODA 



