Besides these constant and essential elements, the central capsule contains very commonly (but not constantly) some other enclosed structures, viz.:—
4. An internal or intracapsular skeleton.
5. Intracapsular vacuoles or alveoli.
6. Fat-granules or oil-globules.
7. Crystals of different composition.
8. Pigment-granules.
The Extracapsulum, or the outer part of the Radiolarian body is also constantly composed of three essential elements,—
1. The Calymma, or the thick extracapsular jelly-veil, completely enveloping the whole central capsule.
2. The Matrix, or the maternal tissue of the external protoplasm, enveloping immediately the capsule-membrane as a thin continuous layer of extracapsular sarcode (ectosarc).
3. The Pseudopodia, or the very numerous thread-like filaments of protoplasm, which radiate from the matrix; whilst their inner part is enclosed in the calymma, their outer part floats freely in the sea-water.