STRUCTURE OF MOLLUSCA. 3 



skeleton, affording a support to the body, and attach- 

 ment to the muscles ; red blood ; a muscular heart ; 

 a mouth with jaws, one placed above the other ; 

 organs of sight, taste, and smell, seated in the cavities 

 of the face ; never more than four limbs, sometimes 

 none ; and a well-developed nervous system, converg- 

 ing in a brain, contained in a bony chamber forming 

 the skull. 



Great differences are apparent, on the contrary, in 

 the creatures now to be considered. The term mol- 

 lusca, derived from the Latin word mollis^ soft, imports, 

 as will at once be obvious, those animated bodies which 

 possess not the hardness of substance common to other 

 living creatures : the snail and the oyster may be men- 

 tioned as familiar specimens. It is easier to say what 

 the moUusca have not, than what they have. The ab- 

 sence of a true internal skeleton renders their forms 

 variable ; one plan does not run through all. The 

 muscles are attached only to a soft contractile skin, 

 which forms a common envelope, and which, in some 

 groups, secretes a calcareous shell, the dwelling of the 

 living animal. This shell, when composed of two por- 

 tions, or valves, is opened by means of an elastic hinge, 

 and closed by a muscle or muscles passing from valve 

 to valve. 



