GYMNOMYXA. 99 



Amoeba is a type of the single class Rhizopoda in which 

 there are pseudopodia, and of the order Lobosa with blunt 

 or lobose pseudopodia, but there are three other important 

 orders to which we may briefly allude. 



The Heliozoa or sun-animalcules are usually spherical in shape, and 

 are found in freshwater. The pseudopodia are long rays usually stiff- 

 ened with an axial rod of silica. The central mass of protoplasm is 

 vacuolated, and some have a hollow perforated shell like those of the 

 next order. Nearly all are centro-symmetric. 



Fig. 33- — A Heliozoan {Actinophrys sol). 



The entire animal magnified. {Ad nat.) 

 Vacuole. Nucleus. 



Central Axis of. 

 Pseudopodium. 



Pseudopodial 

 Ray. 



Note'the central nucleus and stiffened pseudopodia. 



The second order is that of the Radiolaria. They are marine pelagic 

 organisms of microscopic size and have a siliceous skeleton of isolated 

 pieces called spicules^ or a continuous perforated shell through the holes 

 of which the fine radiating pseudopodia protrude. The main mass of 

 protoplasm has a thin capsule dividing it into central and peripheral 

 portions, and in the peripheral parts there are often found a number of 

 minute algoid bodies called yellow -cells. They live and multiply in close 

 organic unity with the radiolarian. Such a union is termed symbiosis 

 (see Chapter IX. ). Radiolarians are commonly centro-symmetric, but 

 some are axo-symmetric. Countless numbers of them live and die in 

 the pelagic water, and their shells and spicules cover the sea-floor at 

 great depths, constituting radiolarian ooze (Chapter IX. ). 



The third order, Foraminifera., also consists of a vast assemblage 

 of small pelagic organisms. They usually have a shell, made of 

 calcareous, arenaceous or chitinous material. It is often chambered, 



