我们在海边捡到了很多软体动物的壳，比如螺和蛤蜊。

We collected many ___molluscs shells by the sea, including snails and clams.

软体动物在维持淡水生态系统平衡方面起着重要作用。

___Molluscs play a vital role in maintaining the balance of freshwater ecosystems.