Complex sentence: For aquatic gastropods, anterior positioning of he mantle cavity may be useful for preventing sediment getting into the mantle cavity, an event which is more likely with posterior positioning because sediment can be stirred up by the motion of the gastropod.
Split into simpler sentences: For aquatic gastropods the anterior positioning may be useful for preventing sediment getting into the mantle cavity. This is more likely with posterior positioning because sediment can be stirred up by the motion of the gastropod.