Class 1. Gymnomyxa. Order 2. Badiolaria. 



91 



give out free oxygen whicli is required by the hosts for respiratory 

 purposes (symbiosis). 



The Radiolaria are exclusively marine; they are frequent in 

 the open sea, and live at very various depths, though chiefly near the 

 surface, where, especially in warmer zones, they occur in enormous 

 numbers and in a great variety of forms. Like the shells of the 

 pelagic Rhizopods, the silicious skeletons of the Radiolaria also sink 

 to the bottom, where they form the chief constituent of the extensive 

 deep sea deposits found in some regions. 



Fig. 50. 



Actinosphwnum 

 Eichornii. k nu- 

 cleus, n food par- 

 ticles, V vacuoles. 

 (After K.Hei twig). 



,. 





A small groiip of Protozoa, tlie Heliozoa is found in fresli water, rarely • 

 m the sea. They differ from the Radiolaria in the ahsence of a central capsule, 

 but in most othei' respects agree with this group. When a skeleton is 

 pi-esent, it is silioioiis (trellissed spheres or spines) ; the pseudopodia are i-adially 

 disposed. Of the freshwater forms, Actinosplaerium Eichhomii, which attains 

 the size of a pin's head, may be mentioned. The protoplasm is very vesicular, 

 and contains numerous nuclei. There is no silicious skeleton, but the 

 pseudopodia ai-e suppoi-ted by firm axial tlueads of organic material. 



Class 2. Infusoria {Clliata). 



The Infusoria possess a thin external layer (ectosarc) of a firmer 

 consistency than the rest of the protoplasm (endosarc), and in 

 consequence cannot protrude pseudopodia. The body, which is 

 usually round, oval, or somewhat more elongate, is, however. 



