SECTION XL PHYLUM MOLLUSCA 



GROUPED together in the Phylum Mollusca are a large 

 assemblage of animals exhibiting as great a diversity in their 

 structure as is observable among the Arthropoda. The animals 

 popularly known as " Shell-fish," such as the Mussels, 

 Oysters and Scallops, the Whelks, Limpets and Snails, 

 together with the Cuttle-fishes and many others, are com- 

 prehended within this extensive phylum. If we compare a 

 Mussel, a Whelk, and a Cuttle-fish, we may at first experi- 

 ence a difficulty in finding a sufficient number of features 

 common to all three to justify us in placing them together in 

 one phylum. They are all unsegmented, and are devoid of 

 the continuous enclosing crust and of the jointed append- 

 ages of the Arthropoda ; and they all possess, in different 

 forms, a calcareous shell, in relation to which is a specially 

 modified area of the skin, the mantle : but it is only on a 

 careful analysis and comparison of the various parts that 

 we are enabled to arrive definitely at the conclusion that 

 they all present us with modifications of the same general 

 plan of structure. 



Five classes are comprised in the phylum : (i) the 

 Pelecypoda, or bivalved shell-fish, such as Mussels, Cockles, 

 Oysters, Scallops, etc., (2) the Amphineura, (3) the Gastro- 

 poda, including the univalved Shell-fish, such as Periwinkles, 



