MARINE MOLLUSCS 255 



CLASSIFICATION OF THE MOLLUSCA 



LAMELLIBRANCHIATA Plate-gilled. Headless, usually en- 

 closed in bivalve shell. 

 Section SIPHONIDA Mantle lobes more or less united to form 



tubular siphons. 



Families Pholadidte, Ga&trochanidce, Anatinida;, Myacidce, 

 Solenidce, Tellinidce, Mactridce, Veneridie, Cyprinidfc, 

 Liicinidce, Cardiadte, &c. 

 Section ASIPHONIDA Mantle lobes free or nearly so. No true 



siphons. 



Families Arcadce, Mytilidce, Avictilidce, Ostreidce, &c. 

 Class CEPHALOPHORA Head-bearing. Usually enclosed in a uni- 

 valve shell. 



Section PTEEOPODA Wing-footed molluscs. 

 Section GASTEEOPODA Stomach-footed molluscs. 



Order Nucleobranchiata Viscera form a nucleus on the back. 

 Order Opisthobranchiata Shell generally absent. Gills more or 



less exposed. 



Section NUDIBRANCHIATA Naked gills. 

 Section TECTIBBANCHIATA Gills covered by shell or mantle. 

 Order Pulmonifera Lung-breathers. Terrestrial. 

 Order Prosobranchiata. 



Section HOLOSTOMATA Aperture of shell entire (sea snails). 

 Families Chitonidce, Dentaliadce, Patellidce, Calyptrcsidce, 

 Fissurellidce, Haliolidce, Turbinidce, Littorinidce, Turri- 

 tellidcB, CeritliiadcB, Pyramidellidce, Naticidce, &c. 

 Section SIPHONOSTOMATA Possess a true siphon. Carnivorous. 



Families Cyprceidce, Conidce, Buccinidce, Muricidce, &c. 

 Class CEPHALOPODA Sucker-bearing arms round the mouth. 

 Order Dibranchiata- Two gills. 

 Section OCTOPODA Eight arms. 



Families Argonautidce, Octopodidce. 

 Section DECAPODA. 



Families Teuthidcs (Calaniaries, Squids), Sepiada, &c. 

 Order Tetrabranchiata Four gills (containing Nautilidce). 



