418 PROTOZOOLOGY 



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 the pseudopodia are withdrawn, the alveoli burst, and the organisms 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, although they are, 

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

 in intracapsular cytoplasm, and in Tripylea they are absent alto- 

 gether. They are spherical bodies, about 15/i in diameter, with a 

 cellulose wall, 2 chromatophores, a pyrenoid, starch, and a single 

 nucleus. They appear to multiply by fission. These bodies are con- 

 sidered as zooxanthellae (p. 215). In the absence of organic food 

 material, the Radiolaria live probably by utilizing the products of 

 holophytic nutrition of these symbiotic 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 pseudo- 

 podia and produce circular groups of short, rod-like bodies clustered 

 around each of the radial spines (Fig. 195, c). They connect the pe- 

 ripheral portion of the body with the pseudopodial covering of the 

 spicule and possess a great contractile power, supposedly with hy- 

 drostatic function (p. 54). 



The skeletal structure of Radiolaria varies considerably from sim- 

 ple to complex and has a taxonomic value. The chemical nature of 

 the skeleton is used in distinguishing the major subdivisions of the 

 order. In the Actipylea it seems to be made up of strontium sul- 

 phate, while in the three other groups, Peripylea, Monopylea, and 

 Tripylea, it consists fundamentally of siliceous substances. The 

 skeleton of the Actipylea is sharply marked from others in form and 

 structure. The majority of this group possess 20 rods radiating from 

 center. The rod-shaped skeletons emerge from the body in most 

 cases along five circles, which are comparable to the equatorial, two 

 tropical and two circumpolar circles of the globe, which arrangement 

 is known as Miiller's law, since J. Miiller first noticed it in 1858. 



The life-cyle of the Radiolaria is very incompletely known (Fig. 

 194). Binary or multiple fission or budding has been seen in some 



