In most vertebrates the skeleton is made from bone (calcium phosphate); among invertebrates it is more varied. Calcite or aragonite in various forms is common in such groups as the brachiopods (Brachiopoda) and molluscs (Mollusca). Normally the invertebrate skeleton is made up from several layers, and often each layer has a distinctive structure. The main, calcified portion of the shell is called the ‘ostracum’; an outer layer, the ‘periostracum’ (made from layers of protein), disappears after death. Calcitic skeletons also occur in corals and Bryozoa. Echinoderms (Echinodermata) have skeletons made up of a number of elements. Each element is permeated with living tissue but the hard material is calcite, which forms in optical continuity to form a single crystal. Chitin (a hydrocarbon related to cellulose) is the principal component of insect cuticle. It has been assumed that chitin impregnated with calcium carbonate forms the exoskeletal material in trilobites (Trilobita), but although organic material occurs its nature is still unknown. Similarly, recent studies of graptolite (Graptolithina) skeletal material have shown that the material is not chitin but a scleroprotein (a fibrous, insoluble protein). In some simple animals, e.g. Radiolaria and some sponges (Porifera), the skeleton is composed of opaline silica.