<P> The endoskeletons of echinoderms and some other soft - bodied invertebrates such as jellyfish and earthworms are also termed hydrostatic; a body cavity the coelom is filled with coelomic fluid and the pressure from this fluid acts together with the surrounding muscles to change the organism's shape and produce movement . </P> <P> The skeleton of sponges consists of microscopic calcareous or silicious spicules . The demosponges include 90% of all species of sponges . Their "skeletons" are made of spicules consisting of fibers of the protein spongin, the mineral silica, or both . Where spicules of silica are present, they have a different shape from those in the otherwise similar glass sponges . </P> <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>

Name three organs of the body a skeleton protects