232 TlMEHRI. 
arranged in plates (lamella?). To this class belong the 
oysters, mussels, cockles, scallops, etc., of economic im- 
portance. A few bivalves — such as the lamp-shells, 
among which are to be found some of the most ancient 
living forms — are known, which belong to another group 
(Brachiopoda) , none of which are in the colle6tion. 
(2.) The univalve shells, technically forms of the class 
Gasteropoda, so called from the fa6l that these animals 
seem to walk on their stomachs. To this class belong 
the whelks, periwinkles, limpets, snails and spiral shells 
generally. Sometimes the shell is much reduced and 
scarcely visible, as in the slugs ; at other times it forms 
a distinct simple cone, as in the limpets ; while again it 
may consist of eight distin6l pieces, as in the ancient 
mail-shells, (Chiton). A few forms of this class instead 
of breathing by means of gills, breathe by means of a lung- 
like chamber, and are terrestrial (slugs and snails). 
(3.) The flat-spiral shells, technically forms of the 
class Cephalopoda, so called from the he^d being covered 
up by the foot-like or arm-like portion. To this class 
belong the two forms of nautilus, the octopus, and the 
other cuttle-fishes. Many of these possess no true shell, 
but only an internal bone, known as the cuttle-bone. In 
those that have true shells, as in the pearly-nautilus, the 
shell is divided by cross partitions into chambers made 
and occupied successively as the animal grew in size. 
The animal thus lives only in the outermost chamber, the 
others being empty. 
The animals of all these groups are known by the 
general name of Molluscs, or soft-bodied animals. They 
have no backbone or hard internal supporting tissue, and 
are destitute of the joints or rings found in the worms, 
