2 THE VOYAGE OF H.M.S. CHALLENGER. 



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 extracap- 

 sular 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. 



Besides these three constant and .essential elements, the extracapsulum contains very 

 commonly (but not constantly) some other enclosed structures, viz. : 



4. An external or extracapsular skeleton. 



5. Extracapsular vacuoles or alveoli. 



6. Fat-granules or oil-globules. 



7. Pigment-granules or a peculiar large body of dark extracapsular pigment, the 



" phseodium." 



8. "Xanthellse" or " zooxanthellee," peculiar yellow cells, which contain starch 



and are unicellular yellow Algse, living as " Symbiontes " in true Symbiosis 

 with a great many Eadiolaria. 



The Nucleus of the Eadiolaria is a large true simple cell-nucleus, originally a solid 

 spherical, roundish or longish body of nuclein. It is placed either in the centre of the 

 capsule (in most Peripylea) or excentrically (in most other Eadiolaria). Originally solid, 

 the nucleus is commonly differentiated later into an outer dense nuclear-membrane and 

 an inner softer or fluid content ; either with one single nucleolus or with a variable 

 number of nucleoli. Originally always simple, the nucleus becomes afterwards constantly 

 divided into numerous small nuclei, each of which, together with a part of the surrounding 



