The order Radiolaria was founded by Muller to include the Polycystina, the Acanthometrina, and the Thalassicollida, to which Dr Carpenter adds Actinophrys and its allies, chiefly on account of the form of the pseudo-podia, the latter forming a special group, to which the name of Heliozoa may be given. Most of the Radiolaria are marine, and the few forms which have been described as occurring in fresh water, are probably best referred to the Heliozoa.
The order Radiolaria may be defined as comprising those Rhizopods which generally possess a siliceous test or siliceous spicules, and are provided with pseudopodia which stand out like radiating filaments, and occasionally run into one another. All of the typical Radiolaria possess a central membranous or chitinous capsule surrounded by an envelope of sarcode. The extra-capsular sarcode generally contains a layer of yellow cells, which are composed partly of starch. If we except Actinophrys and its allies which cannot be regarded as typical members of the order - no contractile vesicle is present. In the aberrant Myxobrachia, also, the characteristic radiating pseudopodia are absent, the organism being furnished with from one to sixteen arm-like processes of sarcode, the clavate ends of which enclose numerous calcareous bodies (coccoliths and coccospheres), which may, however, be simply taken in as food.
The protoplasm of the body of a Radiolarian consists typically of a central and a peripheral portion, of which the former is enclosed in a porous, membranous, or chitinous capsule, while the latter is surrounded by a gelatinous investment. In the often brightly coloured extra-capsular sarcode are scattered the yellow cells above alluded to, which some regard as being truly of a parasitic nature. The pseudopodia (fig. 17), are numerous, filamentous, radiately disposed, and sometimes anastomosing. Skeletal structures, in the form of spicular, radiating spines, or fenestrated shells, may be developed in either the extra-capsular or the intra-capsular sarcode, or in both. These skeletal structures may be wanting; but, when present, they are almost always siliceous, rarely horny, never calcareous. The animal is usually simple, varying in size from 1/600 to 1/20 inch, or rarely more, but in other cases (Collozoum) colonies are formed, which may reach two inches in diameter. Reproduction is often by fission; but in other cases the intracapsular sarcode breaks up into minute germs or zoospores, each of which possesses a nucleus and a flagellum.
Fig. 17. - Eucecryphalus Schultzei, with the pseudopodia extended, showing the perforated siliceous test and the lobed protoplasmic body. After Kolliker. (The author is indebted to the kindness of Professor Mivart for the use of this engraving.)
The following are the more important groups of the Radio-laria: