Conch shell symbolises the deep, far-reaching and melodious sound of the Dharma teachings, which being appropriate to different natures, predispositions and aspirations of disciples, awakens them from the deep slumber of ignorance and urges them to accomplish their own and others' welfare. This conch is kept in 8th offering bowl in the series of eight offering bowls where it represents the music.
It is because conch is the only musical instrument which plays sound without even being played.