PHYLUM MOLLUSCA 



Classes 



1. Lamellibranchia. 



2. Gasteropoda 



3. Scaphopoda. 



4. Cephalopoda 



Sub-Classes 



1. Isopleura. 



2. Anisopleura. 



Orders 



1. Tetrabranchia. 



2. Dibranchia 



Orders 



1. Streptoneura 



(Prosobrancbia). 



k 2. Euthyneura. 



Sub-Orders 

 fl. Nautiloidea. 

 1 2. Ammonoidea. 



fl. Decapoda. 

 [2. Octopoda. 



The majority of the molluscs (oysters, whelks, cuttlefish, 

 etc.) are marine, but some live on land, others in fresh- 

 water. Unlike the worms and arthropods, they are 

 unsegmented animals, and they bear no serially repeated 

 appendages. Typically the body is bilaterally symmetrical, 

 and there is consequently a repetition of the same organs 

 on each side ; but in most gasteropods this symmetry is 

 more or less completely lost. From the dorsal surface arises 

 a fold of the skin forming what is known as the mantle ; 

 this generally secretes a calcareous shell, consisting of one 

 or two (occasionally more) pieces. On the ventral surface 



