Radiolarians can be distinguished from their fresh- 

 water counterparts, the heliozoans, by the definite 

 membrane that separates the outer highly vacuo- 

 lated protoplasm from the inner more granular pro- 

 toplasm — though these are continuous with each 

 other through pores in the membrane. The outer 

 layer is gelatinous, with large fluid-filled vacuoles 

 (none of them contractile) that give it a frothy ap- 

 pearance; and it usually contains yellow bodies, 

 which are thought to be flagellates that exchange 

 favors with their hosts. 



When the weather grows rough or the temperature 

 rises too high, the buoyancy of radiolarians is said 

 to be reduced by a withdrawal of the pseudopods and 

 a bursting of some of the fluid-filled vacuoles in the 

 frothy layer. Surface-living animals can thus descend 

 into deeper water, restoring their vacuoles later. 



Radiolarian shells are exquisite lattices of silica. In 

 life, the protozoans that secrete the shells extend stiff 

 pseudopods from many openings. ( Otto Croy ) 



Some species regularly float at great depths (six- 

 teen thousand feet or three miles). In the deeper 

 parts of the ocean, where the calcareous shells of 

 foraminiferans soon dissolve, the bottom ooze may 

 contain siliceous skeletons only: radiolarian lattices, 

 sponge spicules, and diatom cases. When the radio- 

 larian content reaches at least 20 per cent, bottom 

 deposits are called "radiolarian ooze." In the Pacific 

 and Indian oceans such ooze covers almost three mil- 

 lion square miles of ocean bottom. Upon being up- 



lifted, radiolarian beds become sedimentary rock 

 layers on land, and the small but well-preserved ra- 

 diolarian skeletons, like those of the foraminiferans, 

 serve as convenient index fossils for dating rock lay- 

 ers and for guiding oil-well digging operations. Ra- 

 diolarian deposits occur as siliceous inclusions in 

 other rocks, forming flint or chert. And radiolarian 

 skeletons contribute to the abrasiveness of the "Trip- 

 oli stone" used in metal-polishing powders. 



The Spore-Formers 



{Class Sporozoa) 



The sporozoans include some of man's worst ene- 

 mies, the spore-forming parasitic protozoans that 

 cause malaria, various cattle fevers, coccidiosis in 

 chickens, diseases of halibut, salmon, and other 

 fishes, epidemic death in cultivated honeybees and 

 silkworms. Yet one hesitates to hold a grudge against 

 a whole class of animals that is itself so impartial 

 as to parasitize every major group in the animal 

 kingdom, not excepting other protozoans. Each spe- 

 cies of parasite is more or less limited to a specific 

 host or to a few closely related hosts, and the para- 

 site lives within or between the host cells, absorbing 

 food through its body wall. Feeding in this way, 

 sporozoans can take only dissolved food, sometimes 

 that digested by the host but more often the dissolved 

 protoplasm or body and tissue fluids of the host it- 

 self. An animal that lives protected from the external 

 environment and surrounded on all sides by mate- 

 rials for abundant feasting has little or no need to 

 move about, and the adult or main feeding stage of 

 sporozoans has no external organs of locomotion. 

 Besides this negative criterion, which is used to sep- 

 arate them from other protozoans, all sporozoans 

 share the habit of producing very large numbers of 

 spores as transfer stages to new hosts, and this has 

 suggested the name of the group. The young transfer 

 stage produced by sporulation is usually enclosed in 

 a resistant wall, but in the blood-inhabiting species, 

 like the malarial parasite, the spores are naked and 

 they are never exposed to the rigors of the external 

 world, being transferred from one final host to an- 

 other by a blood-sucking intermediate host. The life 

 cycles of sporozoans can be extraordinarily complex, 

 involving both asexual and sexual processes, each of 

 these with one or more cycles of multiple fission. 

 The nucleus splits repeatedly by rapidly ensuing di- 

 visions, and each new nucleus becomes surrounded 

 by a tiny share of the protoplasm, so that when the 

 cell finally breaks up there are as many ofl'spring 

 as there were nuclei. The simultaneous hatching of 

 billions of slender parasites in each such cycle is 



[ continued on page 49 ] 



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