MOLLUSCA CONSIDERED AS A BRANCH. 391 



shell, and withdraws itself wholly within the same asjain, 



t* ^j 



at will. 



The kinds of Mollusks are very numerous, not less than fifteen 

 thousand. They abound in the sea, on the marshes, in pools, streams, 

 ponds, and lakes, and on the land ; and they are full of interest when 

 we study them, and all serve important purposes. They are the food 

 of many other animals. The Right Whale feeds upon small kinds 

 which swim freely in the open sea ; the Cod and Haddock and many 

 other useful fishes fatten upon those gathered near or on the bottom ; 

 and Sea-birds feast upon those left bare by the tide. Man reckons 

 the Oyster, Clam, and Scallop among his choicest dishes ; and in 

 seasons of scarcity the poor inhabitants on many a sea-coast depend 

 upon Mollusks for a large part of their daily food. These animals also 

 furnish the bait for all the extensive fisheries of the North Atlantic. 

 Some kinds yield rich dyes. The celebrated Tyrian purple of the 

 ancients was obtained from shell-fish, as already stated. 



The shells of Mollusks are limestone or carbonate of lime. Pearly 

 within, and even without when polished, and of soft and delicate 

 colors, they are often exceedingly beautiful, and are eagerly sought 

 for. The child gathers them for toys, and thinks he hears the roaring 

 of the sea as lie puts them to his ear. The savage wears them as 

 ornaments, and some of them as marks of chieftainship. Some 

 kinds are gathered by civilized nations and used instead of money in 

 trading with barbarous tribes. Other kinds are gathered and wrought 

 into almost numberless articles of use and luxury. And the true 

 naturalist, more enthusiastic than all others, traverses sea and land, 

 and cheerfully endures hunger, thirst, and fatigue that his collection 

 of shells may lack neither "Argonaut" nor "Nautilus," "Cone," 

 "Cowry," nor " Wentle-trap," "Helix" nor " Limnseid," "Pecten," 

 "Mother-of -Pearl," nor " Unio," nor any other which will enable 

 him to understand more clearly this department of the animal king- 

 dom, and the works of God as revealed in these wonderful objects. 



In general, the shells of Mollusks are composed of 

 carbonate of lime, and the texture varies with the kinds, 

 as shown when exceedingly thin sections are examined 

 with the microscope (Figs. 526-8). 



The Mollusca are well provided with organs by which 

 food is secured and digested. Their blood is colorless, or 



