CHAPTER XX 



ORDER 2 RADIOLARIA MULLER 



THE RADIOLARIA are pelagic forms found abundantly in the 

 sea. A very large number of species occur in various oceans. 

 In fact a vast area of the ocean floor is 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 forma- 

 tions. Thus this group is the second group of Protozoa impor- 

 tant to geologists. 



The body is mostly spherical in form, although radially or 

 bilaterally symmetrical forms are also encountered. The cyto- 

 plasm is divided into two distinct 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 the use of reagents. Its shape varies accord- 

 ing to the form of the organism. In spherical forms it is spheri- 

 cal, 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 animal 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 and contains reserve food material, while the 

 extracapsular portion is nutritive and hydrostatic in function. 

 The intracapsular cytoplasm is granulated, often greatly vacuo- 

 lated, and is stratified 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 structure, vary among different species and also 

 at different stages of development even in one and the same 

 species. 



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