ZOOLOGY 



Class 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 Eadiolarians occur ia 

 vast numbers on the floor 

 of some seas, forming a layer 

 of sOiceous 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 

 body ; these unite to some 

 extent, nodes of protoplasm 

 being found at the point of 

 union. A streaming of the protoplasm along the pseudopodia, 

 as in Heliozoa and Eeticularia, 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 iu those Protozoa which, like the Eadiolaria, swim 

 near the surface of the sea. 



Some very remarkable bodies, known as yellow cells, are 

 found widely distributed amongst the Eadiolaria. These are 

 small oval yeUow bodies, only found in the extracapsular pro- 

 toplasm. They were formerly regarded as part of the body of 



Fig. 16. — Radiolarian ooze from 4475 

 fathoms in Central Pacific. 



