SECTION XL PHYLUM MOLLUSCA 



GROUPED together in the Phylum Mollusca are a large as- 

 semblage of animals exhibiting as great a diversity in their 

 structure as is observable among the Arthropoda. The ani- 

 mals popularly known as " shell-fish," such as the mussels, 

 oysters, and scallops, the whelks, limpets, and snails, to- 

 gether with the cuttle-fishes and many others, are compre- 

 hended within this extensive phylum. If we compare a 

 mussel, a whelk, and a cuttle-fish, we may experience a 

 difficulty in finding a sufficient number of features common 

 to all three to justify us placing them together in one phylum. 

 They are all unsegmented, and are devoid of the continuous 

 enclosing crust and the jointed appendages of the Arthro- 

 poda ; and they all possess, in different forms, a calca- 

 reous shell, with, in relation to it, 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 Pele- 

 cypoda, 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, 

 whelks, snails, slugs, etc. ; (4) the Scaphopoda or elephant's 

 tusk shells; 1 and (5) the Cephalopoda, including the cuttle- 

 fishes, squids, octopi, and nautili. 



1 Not further referred to in this work. 

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