RADIOLARIA 255 



A thin assimilative layer, or matrix, surrounds the central 

 capsule. In Tripylea, waste material forms a brownish mass, 

 known as the 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 vertical movement of some Radiolaria is due to the forma- 

 tion and expulsion of a fluid which consists of water saturated 

 with carbon dioxide. Under ordinary weather and temperature 

 conditions, the interchange between the alveoli and the ex- 

 terior 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 in extraordinary tempera- 

 ture, the pseudopodia are withdrawn, the alveoli burst, and 

 the organisms descend into deeper water, where the alveoli 

 are formed again. 



The Radiolaria feed on microplankton such as copepods, 

 diatoms, and other Protozoa. The food is taken in through the 

 pseudopodia and passed down into the deeper region of the 

 calymma where it is digested in a food vacuole. The Radio- 

 laria 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 the intracap- 

 sular cytoplasm, and in Tripylea they are absent altogether. 

 They are spherical bodies, about 15 microns in diameter, with 

 a cellulose wall, two chromatophores, a pyrenoid, starch, and 

 a single nucleus. They appear to multiply by fission. These 

 bodies are considered as Zooxanthellae (p. 93). In the absence 

 of organic food material, the Radiolaria use the products of the 

 holophytic nutrition of these symbiotic organisms. 



The axopodia arise either from the extracapsular cytoplasm 

 or from the intracapsular 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. 103, c). They connect the peripheral portion of 

 the body with the pseudopodial covering of the spicule and 



