40 INTRODUCTION TO CONCHOLOGY. 



masonic art is more fully and variously developed. They 

 equally inhabit sea and land; some are even found sus- 

 pended by a peculiar kind of net-work to the boughs of 

 trees ; others inhabit rivers and stagnant pools. Their heads 

 and eyes are distinctly formed, and they move readily by 

 the aid of a contractile and expanding disc. 



Pteropods include a limited class of small, twihght, deep- 

 water swimmers, which move readily, as their name indicates, 

 by means of a pair of wing-like fins. These creatures 

 produce a small brittle shell, resembhng glass. 



Lamellibranchiates are next in the descending scale 

 of organized beings. They form an extensive tribe, whose 

 mantle is divided into two lobes, each with a separate piece 

 of shell, connected by a horny hgament. The brancJiicB, or 

 breathing organs, are arranged in tliin plates ; and, strange 

 to say, the group is uniformly headless : a considerable 

 number are even incapable of motion. 



Brachiopods, last of the molluscous series, have also a 

 bivalve shell, and are uniformly without a head. This 

 group is parasitic; their peculiarity consists in ha\ing a 

 pair of spiral arms, and they are differently placed within 

 the shell. 



