Simplify the following text for a Grade 14 native English-speaking person.
Original: Like all echinoderms , sea cucumbers have an endoskeleton just below the skin , calcified structures that are usually reduced to isolated microscopic ossicles ( or sclerietes ) joined by connective tissue .
Simplified: Like all echinoderms , sea cucumbers have an endoskeleton just below the skin , calcareous structures that are usually reduced to isolated ossicles joined by connective tissue .