Final answer:
In sea urchins and sand dollars, ossicles are fused to form a 'test,' which is a hard, calcareous internal shell that supports and shapes the organism.
Step-by-step explanation:
Sea Urchins and Sand Dollars Ossicles
In sea urchins and sand dollars, ossicles, which are small pieces of skeleton in echinoderms, are fused to form a rigid structure known as a test. The test is a continuous internal shell that provides support and shape to these animals. Sea urchins, belonging to the class Echinoidea, have their ossicles fused to create this hard, calcareous endoskeleton. It contributes to their hemispherical or flattened appearance and is crucial for their locomotion and protection. It features five rows of tube feet that are instrumental in their movement and in keeping the body surface clean by extending through pores in the test.