TOO THE MOLLUSCA 



other Gastropods in being clad in a suit of mail consisting of eight 

 shelly plates which overlap, enabling the molluscs, when alarmed, 

 to roll themselves up in the same way as a woodlouse or armadillo 

 bug. Univalve shells vary remarkably in form, size, and colour, 

 ranging in almost countless modification and variation from the 

 simple tent -shaped shell of the Rock Limpet to the wonderfully 

 sculptured and twisted Wing Shells and Harp Shells of the Tropic 

 seas, and include some of the most beautiful specimens of Nature's 

 handiwork. 



The most common form of univalve is a more or less elongated 

 cone, coiled round and round on an axis, varying from the long, 

 tapering Turret Shells (Terebra), which have twenty or more whorls, 

 to the wide, flattened Ear Shells (Haliotes), which consist practic- 

 ally of an abnormally large body whorl. The majority of spiral 

 shells are dextral, or right-handed, but cases of sinistral or left- 

 handed shells are not remarkably uncommon. These " reversed " 

 shells, as they are called, may occur as monstrosities among species 

 which are normally dextral, or the particular genus or species may 

 be normally sinistral ; while in some cases the shells may be 

 indifferently either one or the other. 



Gastropod Molluscs, with a few exceptions, are unsymmetrical, 

 the body being coiled up in a spiral within the shell, and the 

 breathing organs are developed only on the right side, except, 

 of course, in sinistral shells, when the reverse is the case. 



Shells are composed of pure carbonate of lime, a very small 

 proportion of phosphate of lime, and an organic base called con- 

 chiolin a substance allied to chitin. They are formed by the 

 mantle, the fleshy cloak which envelops the little mollusc. The 

 shelly matter is deposited in layers by the margin of the mantle, 

 for although the whole of the mantle is provided with shell-depositing 

 glands, which are used to thicken the inner layers of the shell 

 and to repair any damage it may sustain, such as a fracture or a 

 small hole, the margin alone enlarges the shell, and contains 

 the pigment cells which give the shells their varied and delicate 

 tints. 



Many univalve shells are provided with a thin, flat plate, 

 called the operculum, which partly or entirely closes the aper- 

 ture when the animal is retracted. The operculum is carried on 

 the dorsal side of the foot, so that in crawling it does not impede 



