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SYSTEMATIC DISTRIBUTION. 



Mollusca is the name given to that large division of 

 the animal kingdom which is characterised by having 

 a soft fleshy body, destitute both of a bony skeleton 

 supporting jointed limbs, and of a hard ringed skin. 



They are covered with a muscular coat, called a 

 mantle, endued with a glairy humour, and are gene- 

 rally furnished with a calcareous envelope called a 

 shell, which is secreted by this coat for the protection 

 of the body or of the more vital organs of the animal. 



They are generally elongate, walking on a single 

 central foot or disk, and furnished with one or more 

 pairs of organs on the head and sides. Their ner- 

 vous system, which furnishes the most distinctive cha- 

 racter of the larger groups of the animal kingdom, 

 merely consists of a certain number of medullary 

 masses distributed to different parts of the body ; one 

 of the masses being placed over the gullet, and enve- 

 loping it like a collar. 



This division of the animal kingdom is subdivided 

 into five classes in the following manner : — 



A. Crawling on a foot placed under the body. 

 I. Gasteropodes, which have a distinct head, furnished 

 with eyes and tentacles, and are usually protected 

 by a conical spiral shell. 



