Complex sentence: The bivalve animal is a rare creature that spends its life inside an elephant tusk-like hard shell made of calcium carbonate and has a protective cap over its head which reabsorbs to burrow into the mud for food.
Split into simpler sentences: The bivalve animal is a rare creature that spends its life inside an elephant tusk-like hard shell made of calcium carbonate. It has a protective cap over its head which it reabsorbs to burrow into the mud for food.