240 MOLLUSCA. 



are terrestrial, and breathe the air ; but the greater 

 part live entirely in the water, from which they 

 derive their nutriment, and in which they breathe by 

 the aid of branchiao, or gill-like appendages. Those 

 which are terrestrial are seen in our gardens, pastures, 

 and plantations ; on the trunks and stems of trees, 

 and in moist and shady places ; while multitudes of 

 the aquatic species are to be found in the seas both 

 of tropical and arctic regions, as well as in those 

 which environ our own islands. Others dwell in 

 lakes and ponds of fresh water, or live at the bottom 

 of rapid streams : some are amphibious. In short, 

 they may be said to be universally distributed 

 wherever circumstances permit of their existence. 



The number of species already in museums pro- 

 bably reaches 8,000 or 10,000. There are cabinets 

 of marine shells, bivalve and univalve, which contain 

 from. 5,000 or 6,000, and collections of land and 

 fluviatile shells which count as many as 2,000. The 

 total number of Mollusks, therefore, probably exceeds 

 15,000 species. The Mollusca can only be studied 

 properly in a living state ; it is only then that they 

 develop their form and true appearance by un- 

 folding their different organs, which in the dead 

 animal are always shrunk, retracted, collapsed, or 

 disfigured to such an extent that there is no possi- 

 bility of delineating them, insomuch, indeed, that the 

 same individual has again and again been described 

 under various names, as belonging to different species. 



When we call to mind the incalculable numbers 

 of these creatures that crawl on the bottom or swim 

 in the bosom of the ocean, and that everywhere 

 abound on dry land, it is evident that their im- 

 portance in creation must be great, beyond human 

 speculation. They are the frequent victims of the 

 indiscriminating and almost insatiable appetite of 

 fishes, and from the stomach of a cod or a flounder 

 you may procure many a shell, not otherwise so 

 easily obtainable. They constitute the principal 

 food of innumerable birds and reptiles. They 



