RADIOLARIA 



31 



Radiolarians are typically planktonic— floating in the waters of the 

 open ocean. They present a superficial resemblance to Heliozoa through 

 the presence of the slender, radiating, pseudopodia associated with the 

 floating habit but in this case the pseudopodia are usually devoid of 

 any axial filament. The protoplasm is as a rule divided into an outer 

 and an inner portion by a membranous central capsule (Fig. 12, c.c) with 

 wide perforations the arrangement of which differs in different members 

 of the group. The internal protoplasm is dense and granular and contains 

 the nucleus. The external protoplasm on the other hand is reduced 

 to a sparse network between large vacuoles containing watery fluid 

 which as in the pelagic 

 Foraminifera serve to 

 bring the creature to the 

 same specific gravity as 

 the sea-water so that it 

 hangs suspended with- 

 out any tendency to 

 rise or sink. In stormy 

 weather the vacuoles 

 shrink, the specific 

 gravity is increased, 

 and the creature sub- 

 sides to below the limit 

 of dangerous wave- 

 action. 



A most charac- 

 teristic feature of the 

 Radiolarian is its skele- 

 ton, composed as a rule 

 of glassy silica. This 



may take the form of radiating rods of silica or, in addition to or 

 instead of these, a lattice-work of sihca is laid down in the external 

 protoplasm (Fig. 12, sk) just below its surface. As growth continues 

 the surface of the external protoplasm becomes further removed from 

 the lattice-work and after a time another lattice-work is laid down, 

 and then another and so on until there may be a number of clear flinty 

 lattice-work shells arranged concentrically one within the other (cf. Fig. 

 13, left-hand figure). These skeletons of Radiolaria (Fig. 13) are in 

 their marvellous variety of form and pattern amongst the most beautiful 

 of microscopic objects. 



There remains to be mentioned a remarkable feature which, though 



Fig. 12. 



Diagram illustrating the structure of a Radiolarian. c.c 

 Central capsule ; N, nucleus ; ps, pseudopodium ; s.c, symbiotic 

 cells : sh, skeleton ; vac, vacuole. 



