PROTOZOA. RADIOLARIA 



29 



substance known as acanthin. The form of the skeleton 

 varies considerably (fig. 6) ; it may be entirely outside the 

 central capsule or partly within, and consists either of 

 isolated spicules, or of a lattice-like or reticulate structure 

 of varying shape, frequently with projecting spines. 



The Radiolaria do not live in fresh water, but they 

 have a very wide distribution in the sea, where they are 

 found in all climates and at all depths, showing the 

 greatest variety of form in tropical regions. In some of 

 the deeper parts of the Pacific and Indian Oceans the 



mm* 



to3 





~ doom/ 



Fig. 6. Fossil Radiolaria. A, Lithocampe tschernyschewi, Devonian. 

 B, Trochodiscus longispinus, Carboniferous. C, Podocyrtis schom- 

 burgki, Barbados Earth (Tertiary). All largely magnified. 



empty shells of these animals settle and accumulate on the 

 sea-bottom, forming a siliceous deposit known as ' Radio- 

 larian ooze.' Only those Radiolaria in which the shell 

 consists of silica are preserved as fossils. 



Cayeux has described as Radiolaria some bodies found 

 in the Pre-Cambrian rocks of Brittany; they are much 



