Chapter 23 

 Order 2 Radiolaria Mtiller 



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 mem- 

 branous 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 addition 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 ellips- 

 oidal, 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 intracap- 

 sular portion of the body is the seat of reproduction, while the extra- 

 capsular region is nutritive and hydrostatic in function. The intra- 

 capsular cytoplasm is granulated, often greatly vacuolated, 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 vesidular 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. 



A thin assimilative layer, or matrix, surrounds the central capsule. 

 In Tripylea, waste material forms a brownish mass known as phaeo- 

 dium, 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 cy- 

 toplasm. Brandt showed that the vertical movement of some Radio- 

 laria is due to the formation and expulsion of a fluid which consists 

 of water saturated with carbon dioxide. Under ordinary weather 

 and temperature conditions, the interchange between the alveoli 



417 



