[Radiolaria Polycystina] In Nassellaria: the first (uppermost) segment of the skeleton, which can be either undivided (undivided cephalis;undivided cephalis 2), or divided into lobes or chambers (trilocular cephalis; multilocular cephalis). The cephalis can be well differentiated from the thorax (type of cephalis 2; undivided cephalis;shell structure 2; shell structure 4), or it can be immersed into it partially or totally (types of cephalis).