You may not think so but sponges actually have skeletons. Their skeleton is made up of a network of

You may not think so but sponges actually have skeletons. Their skeleton is
made up of a network of tiny glass like structures called spicules.
Unlike corals, when sponges die their skeletons dissolve back into the
water. While corals leave a solid skeleton for other things to grow on, a
sponge’s skeleton will rapidly break down and become part of the bottom
sediment. (Photograph: Warren Baverstock)