Animals JBefore Man 



the other hand, the familiar slugs are appar- 

 ently without a shell, as this is so rudimentary 

 as to be concealed within the mantle, while the 

 marine forms known as Nudibranchs are quite 

 naked another of the many cautions not to 

 judge animals by their clothes, or lack of them. 



The members of the class Lamellibranchi- 

 ata, or leaf-gilled, may be readily distinguished 

 by having a hinged shell of two parts or valves, 

 whence the common name of bivalves. To 

 this class belong the common oyster, mussel, 

 clam, and other commercially important species. 



The Gastropoda include the so-called uni- 

 valve shells, such as limpets, ear-shells, snails, 

 and top-shells, many of which are spirally 

 twisted ; but some of the members of this class 

 have no shell, and the curious chitons, often 

 placed here, are protected by a covering of 

 several overlapping parts, on the principle of a 

 piece of scale armor. 



The little wing-shells, forming the class 

 Pteropoda, which stands well up in the group, 

 may or may not be protected by a shell. These 

 animals, in spite of their small size, play quite 



44 



