THE ANIMAL KINGDOM. ^7 



foot is an organ made up of bands of muscles, which 

 pass from the neck of the animal, and, protruding from 

 the shell, spread out into two wing-like membranes, 

 acting as a double paddle, by means of which the Ptero- 

 poda swim rapidly through the water. Although there 

 are not many species of these animals, they exist in 

 immense numbers, and form the chief food of the Whale. 



Gasteropoda* are a much more extensive class ; so 

 numerous are they, in fact, that they have been further 

 divided into many sub-classes. The limits of this book 

 will not, however, admit of a detailed account of each 

 of these, and we must therefore, for the present, be con- 

 tent with a general description. 



The name stomach-footed is given to the animals in 

 this class because they walk, or rather trail along, on a 

 fleshy mass or foot, on which their body rests. They are 

 but very slow in their movements, and therefore when 

 any person is inactive or lazy, he is said to be " sluggish," 

 or to " move at a snail's pace." The mantle or skin of the 

 back of some of them is naked and clammy, whilst in 

 others it has the power of forming a shell, called 

 univalve, as it consists of one shell only. To this group 

 belong Snails (Fig. 3), Slugs, Whelks, Wentletraps, 

 Periwinkles, Cowries, Murexes, c. 



If you place a Garden-Snail on a piece of glass, it will 

 soon begin to crawl ; and if you then raise the glass and 

 look at the under surface of it, you will see the action of 

 the foot when in motion. 



Brachiopoda, or Arm-footed Mollusca, form a small 

 group ; they are contained in a pair of shells, which are 

 therefore called bivalve shells. Their general form is 

 something like that of the Cockle, but one of the shells 

 is larger and deeper than the other. Their shells are 



* Caster, the belly ; pans, a foot. 



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