Chapter 22 

 Order 2 Radiolaria MUller 



THE Radiolaria are pelagic in various oceans. A vast area of 

 the ocean floor is known to be covered with the ooze made up 

 chiefly of radiolarian skeletons. They seem to have been equally 

 abundant during former geologic ages, since rocks composed of 

 their skeletons occur in various geological formations. Thus this 

 group is the second group of Protozoa important to geologists. 



The body is generally spherical, although radially or bilaterally 

 symmetrical forms are also encountered. The cytoplasm is divided 

 distinctly into two regions which are sharply delimited by a 

 membranous structure known as the central capsule. This is a 

 single or double perforated membrane of pseudochitinous or 

 mucinoid nature. Although its thickness varies a great deal, the 

 capsule is ordinarily very thin and only made visible after addi- 

 tion of reagents. Its shape varies according to the form of the 

 organism; thus in spherical forms it is spherical, in discoidal or 

 lenticular forms it is more or less ellipsoidal, while in a few cases 

 it shows a number of protruding processes. The capsule is capable 

 of extension as the organism grows and of dissolution at the time 

 of multiplication. The cytoplasm on either side of the capsule 

 communicates with the other side through pores which may be 

 large and few or small and numerous. The intracapsular portion 

 of the body is the seat of reproduction, while the extracapsular 

 region is nutritive and hydrostatic in function. The intracapsular 

 cytoplasm is granulated, often greatly vacuolated, and is strati- 

 fied either radially or concentrically. It contains one or more 

 nuclei, pigments, oil droplets, fat globules, and crystals. The 

 nucleus is usually of vesicular type, but its form, size, and struc- 

 ture, vary among different species and also at different stages of 

 development even in one and the same species. 



A thin assimilative layer, or matrix, surrounds the central 

 capsule. In Tripylea, waste material forms a brownish mass 

 known as phaeodium, around the chief aperture (astropyle) of the 

 capsule. Then there is a highly alveolated region, termed 

 calymma, in which the alveoli are apparently filled with a 

 mucilaginous secretion of the cytoplasm. Brandt showed that the 



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