SUB-KINGDOM VII. MOLLUSCA. 



Classes. 



1. Lamellibranchiata. 



2. Gasteropoda 



3. Scaphopoda. 



4. Cephalopoda 



Orders. 



1. Asiphonida ............. 



2. Siphonida ................ 



1. Polyplacophora. 



2. Prosobranchiata. 



3. Opisthobranchiata. 



4. Heteropoda. 



5. Pulmonata. 



6. Pteropoda. 



:1. Tetrabranchiata 

 2. Dibranchiata. . . . 



Sections. 

 (\. Monomyaria. 

 2. Heteromyaria. 

 \B. Homomyaria. 

 j 1 - Integripalliata. 

 1 2. Sinupalliata. 



Sub- Orders. 



fl. Nautiloidea. 

 (2. Ammonoidea. 

 fl. Decapoda. 

 (2. Octopoda. 



THE majority of the mollusks (Oysters, Whelks, Cuttle- 

 fishes) are marine, but some live on land, others in 

 fresh-water. Unlike the worms and arthropods, they are 

 unsegmented animals and bear no appendages. Typically 

 the body is bilaterally symmetrical and there is con- 

 sequently a repetition of the same organs on each side. 

 But in many gasteropods this symmetry is more or less 

 obliterated. From the dorsal surface there is a fold of the 



