Sea cucumbers (Class Holothuroidea) have elongated radial
axes, and thus have cylindrical bodies whose surfaces are soft
since the skeletal ossicles are small and embedded. Members of
the Class Echinoidea have a shorter radial axis and a
hemispherical shape (sea urchins) or a very short axis and a
disc shape (sand dollars). In these animals, some broad
ossicles fuse to form a rigid endoskeleton from which other
sharp, movable spines project outward for protection.
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