CHAPTER VIII 

 TYPE VI. MOLLUSCA 



The Mollusca (Lat. mollis, soft) include those animals called 

 popularly shellfish, such as snails, clams, oysters, and scallops, 

 and in addition the slugs, scpiids, and cuttlefish. Despite this 

 great variety of form, these animals merely present modifica- 

 tions of a common plan or type, so that the group as it stands 

 to-day is pretty characteristic. Its position in the system of 

 classification is very doubtful, however; the fact that we take 

 it up at this point does not mean that it is more highly organ- 

 ized than the Arthropoda, or even than the Annelida ; this is 

 merely a convenient place to consider the group. 



The Mollusca are bilaterally symmetrical animals, though a 

 torsion of some parts may render the adult more or less asym- 

 metrical. There is no evidence of segmentation in the adult, 

 and so there maybe some reason for placing the type below the 

 segmented animals. There are no jointed appendages. The 

 body is soft and moist ; on the ventral side is a structure, called 

 the foot, which is the chief organ of locomotion, but undergoes 

 great modifications in the different groups. The body is usually 

 more or less completely covered with a shell, either univalve or 

 bivalve; this shell is lined by a fold of the skin, called the 

 mantle, which hangs about the body, and the glands in the 

 mantle secrete the shell, which consists chiefly of calcium car- 

 bonate. The space between the mantle and the body-wall of 

 the animal is the mantle cavity. Often the shell can be seen to 

 consist of three parts : a thin brown layer on the outside, which 

 is chitinous ; a much thicker middle laver, which is limy ; and 

 an inner lining, which is the pearly or nacreous layer, beautifully 

 iridescent. With the exception of the land snails and slugs the 

 members of this order are aquatic, some living in fresh water, 



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