Internal gills—see Stewart's organs
Internal movements of protoplasm, [17]
Interradial plates, of calcareous ring of Holothuria nigra, [566];
of Holothuroidea, [569];
of Synaptida, [569];
of Dendrochirota, [569];
of calyx of Crinoidea, [589];
of Thaumatocrinus, [589];
of Hyocrinus, [590];
of Rhizocrinidae, [591];