Hardened plates of calcium carbonate form its endoskeleton.
Echinoderms have an Endoskeleton made up of 95 % calcium carbonate.
Endoskeleton
Sea Cucumbers.
Echinoderms, such as sea stars and sea urchins, have an endoskeleton made of hard plates called ossicles that lie inside their bodies. Although this endoskeleton is internal, it functions similarly to an exoskeleton by providing support and protection. It allows echinoderms to control their body shape and movement, just like an exoskeleton does for other invertebrates.
There are many. The only phylum that contains invertebrates with exoskeletons are Echinoderms. An example is cuttlefish.
Neither are protostomes. Chordates, Hemichordates, and Echinoderms are all deuterostomes (in that the blastopore forms the anus first). In protostomes, the blastopore forms the mouth first.
Sea Stars otherwise known as Star Fish are Echinoderms and they do not have an exoskeleton, they have an endoskeleton.
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.
In echinoderms, such as starfish, an ossicle is a bony plate providing structure and protection. It is covered by an epidermis and considered part of the hard endoskeleton where the bony (calcite) plates (ossicles) are connected by collagen fibers.
echinoderms, cnidaria and other forms
Endoskeleton
Endoskeleton - their bones are inside their body