ON THE STEUCTUKE OF THE MOLLUSCA. 



The animal kingdom has been divided into four great 

 sections, characterized by peculiarities of organization, and 

 named Vertehrata, Mollusca, Articulata, and Radiata. 



The Vertebrata are distinguished from the rest by 

 having an internal skeleton, composed of bones articulated 

 ^vith each other, a brain and spinal marrow contained within 

 a skull and vertebrae, together with five organs of sense, 

 and an alimentary canal opening at the two extremities of 

 the body, and placed below or before the great trunk of the 

 nervous system. The Mollusca, Articulata, and Radiata, 

 have no internal skeleton, distinctly defined brain, or spinal 

 marrow. The Articulata have the body and limbs 

 divided into segments, or formed of pieces jointed together. 

 The Radiata have a more or less evident disposition of 

 their organs, or of some of them, into a radiating form, so 

 as to spread out from a common centre. 



The Mollusca, so named because generally of soft struc- 

 ture internally {Animalia moUia, soft animals), may be de- 

 fined as follows : — Soft, symmetrical, inarticulated animals, 

 enveloped in a muscular skin, or mantle, which is sometimes 

 bare, but generally has attached to it, externally or inter- 

 nally, a calcareous part, or shell, of one or several pieces; 

 with a complete twofold (or general and pulmonic) circu- 

 lation of white (or at least not red) blood, contained in 

 arteries and veins; respiration of water by branchiae (or 

 gills), or sometimes of air by a cavity on the walls of which 

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