20 ZOOLOGY 
Criass VI. Radiolaria. 
Organisms which are either spherical or with one principal 
axis whose body is divided into a central mass containing 
one or more nuclei and a peripheral portion, by the presence 
of a membrane known as the “central capsule.” This is 
perforated so that the intracapsular protoplasm is continuous 
with the extracapsular protoplasm. A well-developed skeleton, 
in most cases siliceous, is present. This consists either of 
loose siliceous spicules or of a continuous skeleton which 
may take the form of lattice-work spheres, arranged con- 
centrically, and united to one 
another by radial spicules, 
which project beyond the sur- 
face of the body. The skele- 
tons of Radiolarians occur in 
vast numbers on the floor 
of some seas, forming a layer 
of siliceous ooze (Fig. 16). 
The skeleton may be wholly 
outside the central capsule, 
or it may be partially within 
it. Numerous fine pseudo- 
podia radiate around — the 
Fic. 16.—Radiolarian ooze from 4475 body ; these unite to some 
fathomieini@antral Pacific: extent, nodes of protoplasm 
being found at the point of 
union. <A streaming of the protoplasm along the pseudopodia, 
as in Heliozoa and Reticularia, takes place, and granules have 
been seen to circulate between the intra- and extra-capsular 
protoplasm. No contractile vacuole has ever been observed. 
The protoplasm is much vacuolated: a condition commonly 
met with in those Protozoa which, like the Radiolaria, swim 
near the surface of the sea. 
Some very remarkable bodies, known as yellow cells, are 
found widely distributed amongst the Radiolaria. These are 
small oval yellow bodies, only found in the extracapsular pro- 
toplasm. They were formerly regarded as part of the body of 
