CHAPTER VI. 
SIXTH BRANCH OF ANIMALS, 
MOLLUSCA. 
General Characteristics—Mollusks are soft, unjointed 
animals enveloped by a muscular cloak or mantle, generally 
protected by a shell. They have a well-defined nervous 
system, a heart, arteries, and veins through which passes 
colorless blood, a foot for locomotion, and eyes more or 
less developed ; 20,000 living species are known, and 19,000 
fossil. Those with two valves, as the oyster, are called 
bivalves, and those with one, as the snail, univalves. The 
former are called Lamellibranchs, from the folded plate- 
like appearance of their gills. 
Class I.—OvsTERs, etc. (Lamellibranchiata). 
General Characteristics—THE SHELL.—The shell (Fig. 
54) is formed of carbonate of lime, secreted by the edges 
of the mantle, which is divided into two halves on the 
right and left sides, each one secreting a valve. The part 
of the shell where growth commences is called the beak 
(Fig. 54, 2); that where the shell opens, the base, £. The 
portion indicated by the direction of the beaks is the ante- 
rior side ; the opposite, the posterior. Near the beaks is 
the hinge 4, and here the valves join by teeth, c, @d, that 
fit into cavities on the opposite valve. A horny ligament, 
h, connects the valves, always tending to throw them apart ; 
thus, dead clams are always found open. In the interior 
