III. SUBKINGDOM MOLLUSCA. 



General Characteristics. — The MoUuscans (soft- 

 bodied) have yielding tissues ot great contractile power, 

 inclosed in a soft flexible 

 skin called the mantle. ^^' ''^' 



The nervous ganglia are 

 placed irregularly, but all 

 communicate with a larger 

 mass forming around the 

 throat a ring, which may 

 be considered the brain. 

 Except in one class, the 

 muscles are attached to the 

 skin, which is usually pro- 

 tected by a shell consisting principally of carbonate of lime, 

 which in turn is defended by an epidermis of animal matter, 

 to prevent corrosion by the water. The blood is white, and 



Mg. us. 



Nervous Syaiein of a MoUitsk. 



the circulation effected by means of a distinct heart. Mol- 

 lusks are mostly oviparous. 

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