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The osteon is the primary structure that makes up the hard outer layer of bones, which is generally known as â cortical bone .â Cortical bone, or compact bone, is dense, mature bone found in vertebrates such as mammals, amphibians, reptiles, and birds. It is this dense hard layer that gives a person or animal strength.
Skeletal Composition. Echinodermata means spiny skinned.. Surprisingly, the external spines are composed of the same material as the internal skeleton. Calcium carbonate and protein form the rigid inner skeleton, or ossicles, of echinoderms.The exoskeleton is composed of soft, spongelike micro-structures called stereom.he uniform symmetry of echinoderms sets the creatures off from other marine life. All echinoderms are pentameral -- they have limbs in multiples of five. This trait is easily recognizable in starfish and brittle stars with only five limbs.