368 PROTOZOOLOGY 



vertical movement of some Radiolaria is due to the formation 

 and expulsion of a fluid which consists of water saturated with 

 carbon dioxide. Under ordinary weather and temperature condi- 

 tions, the interchange between the alveoli and the exterior is 

 gradual and there is a balance of loss and gain of the fluid, so that 

 the organisms float on the surface of the sea. Under rough weather 

 conditions or at extraordinary high temperatures, the pseudo- 

 podia are withdrawn, the alveoli burst, and the organisms 

 descend into deeper water, where the alveoli are reformed. 



The Radiolaria feed on microplankton such as copepods, dia- 

 toms, and various Protozoa. The food is taken in through pseudo- 

 podia and passed down into the deeper region of calymma where 

 it is digested in food vacuoles. The Radiolaria can, however, live 

 under experimental conditions without solid food if kept under 

 light. This is ordinarily attributed to the action of the yellow 

 corpuscles which are present in various parts of the body, al- 

 though they are, as a rule, located in the calymma. In Actipylea 

 they are found only in intracapsular cytoplasm, and in Tripylea 

 they are absent altogether. They are spherical bodies, about 15/z 

 in diameter, with a cellulose wall, 2 chromatophores, a pyrenoid, 

 starch, and a single nucleus. They appear to multiply by fission. 

 These bodies are considered as zooxanthellae (p. 23-24). In the 

 absence of organic food material, the Radiolaria live probably 

 by utilizing the products of holophytic nutrition of these sym- 

 biotic organisms. 



The axopodia arise from either the extracapsular or the intra- 

 capsular portion and radiate in spherical forms in all directions, 

 as in Heliozoa. In Actipylea, myonemes are present in certain 

 pseudopodia and produce circular groups of short, rod-like bodies, 

 clustered around each of the radial spines (Fig. 168, c). They 

 connect the peripheral portion of the body with the pseudopodial 

 covering of the spicule and possess a great contractile power, 

 supposedly with hydrostatic function (p. 52-53). 



The skeletal structure of Radiolaria varies considerably from 

 simple to complex and has a taxonomic value. The chemical 

 nature of the skeleton is used in distinguishing the major sub- 

 divisions of the order. In the Actipylea it seems to be made up of 

 strontium sulphate, while in the three other groups, Peripylea, 

 Monopylea, and Tripylea, it consists fundamentally of sihceous 

 substances. The skeleton of the Actipylea is sharply marked 



