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. OYSTERS, etc. (Lamellibranchiatd). 



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, a] ; that where the shell opens, the base, k. The 

 portion indicated by the direction of the beaks is the ante- 

 rior side ; the opposite, the posterior. Near the beaks is 

 the hinge b, and here the valves join by teeth, c, d d, that 

 fit into cavities on the opposite valve. A horny ligament, 

 /*, connects the valves, always tending to throw them apart ; 

 thus, dead clams are always found open. In the interior 



