214 The Sarcodina 



ically multinucleate, while the Monopylina and Tripylina are usually 

 uninucleate. The extracapsular cytoplasm is concerned primarily with 

 flotation, capture of food, and digestion. Several layers may be recog- 

 nizable (Fig. 5. 11, D): the sarcomntrix, a so-called digestive layer next to 

 the central capsule; the vacuolated calymma, which is a thick zone in some 

 species; a thin layer outside the calymma; and the zone of axopodia whose 

 axonemes often arise in the sarcoma trix. Food is captured much as in the 

 Heliozoida. Since the size of solitary species ranges from about 50[x to 

 several millimeters, the larger Radiolarida are able to feed on copepods 

 and other small Crustacea, as well as on algae and Protozoa which come 

 in contact with the pseudopodia. 



The "yellow-cells" (zooxanthellae), present in many Radiolarida al- 

 though not in the Tripylina, are more numerous in species with a well- 

 developed calymma. They are typically intracapsular in the Actipylina, 

 extracapsular in other groups. In the living host, these parasites are com- 

 monly spherical to ovoid. After death of the host, they may develop into 

 palmella stages which give rise to flagellates. Certain of these flagellates 

 have been referred to the Dinoflagellida (22, 58). Their reputed status as 

 symbiotes remains somewhat uncertain. 



Some of the Radiolarida, such as Collozoum and Sphaerozoum, are 

 colonial forms (16, 140) in which a number of central capsules are em- 

 bedded in an elongated or more or less spherical mass of extracapsular 

 cytoplasm. In certain species at least, each central capsule contains a 

 number of nuclei. Skeletal elements are often reduced to scattered spic- 

 ules, although lattice-work shells occur in some species. 



Life-cycles. As a result of the difficulties in obtaining adequate material 

 for study, little is known about the life-cycles of Radiolarida. Various 

 accounts in the older literature suggest that the life-cycles may be fairly 

 complex, but more extensive observations are needed. Since some of the 

 shallow-water species will survive in the laboratory for reasonable periods, 

 perhaps the application of techniques which have already been so pro- 

 ductive for Foraminiferida would yield valuable information on Radio- 

 larida. 



Although reproduction has been traced in relatively few species, fission 

 occurs in species with simple skeletal elements. The central capsule is 

 divided, and any skeletal elements are passed on to the two daughter 

 organisms. Fission also has been reported within the helmet-shaped skele- 

 ton of certain Tripylina. One daughter organism retains the old shell; 

 the other leaves and develops a new one. According to Brandt (14), cer- 

 tain Thallophysidae may undergo a complicated plasmotomy which fol- 

 lows dedifferentiation of the adult, and results in a number of small 

 organisms, each with several nuclei. Budding possibly occurs in a few 

 species (15), but the process needs further investigation. 



Evidence for sexual phenomena in Radiolarida is still inconclusive, al- 



