CHAPTER XXI 



THE SNAIL HELIX POMATIA AND 

 HELIX ASPERSA 



THE fresh-water mussel, whose anatomy has been described 

 in the last chapter, is an example of a bilaterally-symmetrical, 

 headless, lamellibranchiate mollusc. It has a median anterior 

 mouth, a median posterior anus, a median dorsal heart en- 

 closed in a pericardial chamber, a pair of auricles, a pair of 

 much modified gills or ctenidia, a pair of excretory organs, a 

 pair of gonads, and a pair of well-developed mantle lobes 

 which secrete the right and left valves of the shell. The snail 

 is an example of a large class of molluscs in which the primitive 

 symmetry of the body has been to a great extent lost through 

 distortion of the visceral mass, with the consequence that the 

 organs of the right side of the body have disappeared. It 

 further differs from the fresh-water mussel and all lamelli- 

 branchiata in having a distinct head-region, a complex rasping 

 organ or odontophore in the buccal cavity, and a shell which is 

 all of a piece and coiled into a spiral. These features are 

 characteristic of the class Gastropoda, to which the snail 

 belongs in common with limpets, ormers, winkles, whelks, sea- 

 slugs, and a host of other forms. Some of the Gastropoda 

 have shells, some have not ; most of them live in the sea and 

 breathe by gills or ctenidia, but others, like the snail, are 

 terrestrial, have lost their ctenidia and breathe air contained in 

 a pulmonary chamber. Some gastropods have lost the ctenidia, 

 excretory organs and gonads of one side of the body, in others 

 the primitive paired arrangement is retained : in fine, the class - 

 exhibits every variety of structure, and it is only by the com- 

 parison of a large number of forms that we are able to form an 

 idea of what may be called the typical Gastropod organisation. 

 It is beyond the scope of this work to enter into the minute 

 detail and comparison necessary to the complete understanding 

 of Gastropod anatomy, but as the common snail does not 

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