70 INTRODUCTION TO CONCHOLOGY. 



net, and, therefore, are rarely souglit for ; but their varied 

 forms, and adaptation to their native habitats, are among 

 the wonders of creation. 



The land species of such moUusks as belong to the class 

 Gasteropoda, offer extreme modifications of depressed growth. 

 The reasons of this are obvious : — their visceral portions are 

 contained within a smaller space than those of their carnivorous 

 brethren ; they have no occasion for teeth and rasped plates, 

 nor a complicated digestive apparatus. It is otherwise with 

 the flesh-eating tribes, and hence they have greater powers 

 of shell-making. A ponderous, massy shell is often the pro- 

 duction of an animal which apparently does not require such 

 a spacious mansion ; but, though large, they have no external 

 ramification, when constructed by terrestrial species, and 

 little variety of sculpture ; whilst such as inhabit the water 

 beautify their homes with ribs and tubercles, with plates, 

 and spines, and fronds. 



Seven distinct orders belong to the series of Gastropods, 

 each of vrhich are characterized by varieties in the respira- 

 tory organs : — 



Order I. Pectinibranchiata. — Including numerous mol- 

 lusks, of '«Iiich the hranchm, the gills or breathing 

 apparatus, are arranged for the most part in parallel 



