<P> The skeleton of the echinoderms, which include, among other things, the starfish, is composed of calcite and a small amount of magnesium oxide . It lies below the epidermis in the mesoderm and is within cell clusters of frame - forming cells . This structure formed is porous and therefore firm and at the same time light . It coalesces into small calcareous ossicles (bony plates), which can grow in all directions and thus can replace the loss of a body part . Connected by joints, the individual skeletal parts can be moved by the muscles . </P> <P> In most vertebrates, the main skeletal component is referred to as bone . Another important component is cartilage which in mammals is found mainly in the joint areas . In other animals, such as the cartilaginous fishes, which include the sharks, the skeleton is composed entirely of cartilage . The segmental pattern of the skeleton is present in all vertebrates (mammals, birds, fish, reptiles and amphibians) with basic units being repeated . This segmental pattern is particularly evident in the vertebral column and the ribcage . </P> <P> Bones in addition to supporting the body also serve, at the cellular level, as calcium and phosphate storage . </P> <P> The skeleton, which forms the support structure inside the fish is either made of cartilage as in the (Chondrichthyes), or bones as in the (Osteichthyes). The main skeletal element is the vertebral column, composed of articulating vertebrae which are lightweight yet strong . The ribs attach to the spine and there are no limbs or limb girdles . They are supported only by the muscles . The main external features of the fish, the fins, are composed of either bony or soft spines called rays, which with the exception of the caudal fin (tail fin), have no direct connection with the spine . They are supported by the muscles which compose the main part of the trunk . </P>

What is the main type of skeletal cartilage