Many organisms belonging to this phylum have a calciferous shell. The bodies are generally very soft and are covered by the hard exoskeleton. They can be found in terrestrial regions as well as in the depths of the seas. You will be amazed to know that the size of the molluscans can range from 20 metres to one millimetre, with a few microscopic animals too. These animals continue to have an important role in the lives of humans.
They are a source of food as well as jewellery. The hard shells are used to make beautiful jewellery pieces. In some regions of the world, these are also raised as pets, even though it sounds bizarre. Pearls that are obtained from bivalves and gastropods are valuable as these pearls are lined with nacre. Natural pearls are formed when a small foreign object gets stuck in between the mantle and shell of the mollusc.
The bivalve molluscs are used as bioindicators of the freshwater and marine environments. But not all molluscans are good for humans. Some are pests like the snails and slugs.