A FEW FACTS ABOUT ZOOLOGY. 



MOLLUSKS, 



A mollusk is a Boft-bodied animal, without internal skele- 

 ton and without joints, covered with a moist skin. In some 

 mollusks the skin is naked — generally it is protected by a 

 shell. The lowest class of mollusks are headless animals ; 

 the oyster is a good example of this class. It has, like most 

 acephala, or animals without heads, a shell composed of two 

 valves, united by a hinge on the back, one of these valves 

 being thick and swollen, while the other is nearly flat. If 

 we lift the shell we find beneath a soft lining or skin covering 

 the whole animal, and called by naturalists the mantle, from 

 the inner surface of which arises a double row of gills form- 

 ing two pendent folds on the sides of the body. At one end 

 of the body these folds do not meet, but leave an open space, 

 which is the aperture we call the mouth. This is the only 

 indication we find to tell us which is the forward part of the 

 body, but it is enough to establish a difference between the 

 front and hind ends, and to serve as a guide in distinguish- 

 ing right and left sUes. In this class of headless animals we 

 place oysters, clams, muscles, and the like. When speaking 

 of them, in reference to the number of their shells, we call 

 them bi- valves. 



A step higher among the mollusks we find snails, slugs, 

 cockles, periwinkles, and the like. The mantle envelops the 

 soft body of these animals, lining their single shell, just as 

 it lined the double shell of the oyster. This class of animals 

 has a fleshy, muscular expansion, on which they move, and 

 which is thence called a foot; except that they move by it, 

 it bears no resemblance to a foot, however. These animals 

 are sometimes without shells; if they possess one, it is a 

 single shell or univalve ; these are sometimes flat, simple 

 shells, as in the limpet; sometimes a plain spiral, as the 

 snail; sometimes elaborate spirals with brilliant hues, as in 

 the cowry. In this class, although there is nothing th it can 

 be properly called a head, the front end of the animal is 



