Complex sentence: Two gonads sit next to the coelom, a small cavity that surrounds the heart and sheds ova or sperm into the coloem, from which the nephridia extract them and emit them into the mantle cavity.
Split into simpler sentences: Two gonads sit next to the coelom, a small cavity that surrounds the heart, into which they shed ova or sperm. The nephridia extract the gametes from the coelom and emit them into the mantle cavity.