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SUB-KINGDOM (TYPE) V.MOLLUSCA. 



SOFT-BODIED, unsegmented animals, usually provided with 

 an exoskeleton. Alimentary canal shut off from the body- 

 cavity. Nervous system in the form of three principal pairs 

 of ganglia, which are reduced to one in the lower types. A 

 distinct heart, and specialised organs of respiration, may or 

 may not be present. Distinct reproductive organs are pres- 

 ent in all, though among the lower forms of the sub-kingdom 

 the production of colonial organisms by continuous gemma- 

 tion is not uncommon. 



7 



Fig. 09. Diagram of the structure of a typical Mollusc (the Common Whelk) f, The 

 muscular " foot ; " op, The operculum ; t, One of the tentacles, or feelers, with an eye at its 

 base ; p, The proboscis, retracted, with the mouth at its extremity ; oe, Gullet ; g, Stomach ; 

 i, Intestine, terminating in the anus ; n n, Salivary glands ; I, The liver and the ovary ; 7t, 

 The heart ; 6c, The gill, contained in a hood of the mantle ; s, Breathing-tube or siphon ; c 

 and c, The main nerve ganglia. 



