[Radiolaria Polycystina] In Spumellaria: the innermost shell of the set of concentric spheres of an actinommid (Stylatractus spp. group; Cromyechinus icosacanthus intro). Sometimes also applied to members of other families with several successively larger shells totally or partially enclosing one another. Shells are numbered from the inside of the skeleton toward the periphery; thus, the second shell is the one located next to- and outside of the first one, and so on.