How do ehinoderms grow?

Answer 1

In echinoderms, instead of having an exoskeleton, there is a thin layer of skin that covers a mesodermal endoskeleton. The skin is made of tiny calcified plates and spines and forms a support system for the contained tissues. Despite robust individual skeletal modules, complete skeletons of echinoderms are rare; echinoderms disarticulate once the surrounding skin rots away. Modular construction is the result of the growth system: it adds new segments at the center of radial limbs, pushing already existing plates outward.