Final answer:
The calcareous plates found underneath the epidermis of echinoderms are called the test.
Step-by-step explanation:
The calcareous plates found underneath the epidermis of echinoderms are called the test. The test is a hard, internal endoskeleton made up of multiple calcium carbonate plates that provide support and protection to the animal. These plates are covered by a thin layer of skin called the epidermis.