CHAPTER IX 



PHYLUM X.: MOLLUSCA 



MOLLUSCA are all soft -bodied animals, usually 

 Characters covered by a continuous calcareous shell ; this is 

 secreted by a special fold of the body-wall, which 

 lines the shell and which is called the mantle. This mantle 

 may cover the whole body in one single piece, or it may 

 hang down on the two sides of the body as two separate 

 flaps or mantle-lobes. These two different conditions of the 

 mantle are correlated with corresponding differences in the 

 form of the shell, which may therefore be univalve or bivalve 

 — a characteristic used as a basis for the classification of the 

 Mollusca, for many other peculiar features are associated with it. 

 The organ of locomotion in all Molluscs is a single, ventrally 

 placed, muscular mass known as the foot, in the upper part of 

 which the viscera are to some extent embedded. As in all 

 animals higher than the Coelenterata, there is an alimentary 

 canal distinct from the body-cavity or coelom, but the 

 latter is much reduced, being represented by the small 

 " pericardium," or chamber in which thejieart lies. 



The Univalve Molluscs include snails, slugs, 

 the'phv'um '^''^^ll^Si limpets, and periwinkles. A special sub- 

 division of these univalve forms contains the 

 cuttlefish, octopus, nautilus, and others adapted to a free- 

 swimming marine life. 



The Bivalve Molluscs include mussels, oysters, and cockles. 



Univalve Molluscs 



Ti/pe : The Common Pond Snail (Limnaea stagnalis). 



The snail, Limnaea stagnalis, is to be found in 

 almost any pond or river, especially where vegeta- 

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