General Morphology o£ the Protozoa H 



spicules, or other materials cemented together over a thin chitinous test. 

 The composition of other foramiferan tests varies from group to group. 

 That of the AUogromiidae is typically chitinous, while the majority of 

 the multichambered tests are calcareous. Siliceous tests also have been re- 

 ported in a few Foraminiferida. In many species at least, the foraminif- 

 eran test is not really external; instead, it is normally enclosed within a 

 thin layer of cytoplasm. 



The simplest skeletons of Radiolarida are represented by scattered 

 siliceous spicules, while the more complicated types are structures unique 

 among the Protozoa (Chapter V). In the Acantharina long spines radiate 

 in definite patterns from the center of the body. To these elements is 

 often added a lattice-work shell, joining and supported by the spines. 

 Siliceous skeletons of other Radiolarida are quite varied in structure. 

 Spherical types may be composed of several concentric lattice-work shells, 

 and sometimes of spicules in addition. Bilateral types, conical forms, and 

 other departures from radial symmetry are fairly common. 



PSEUDOPODIA 



Pseudopodia are temporary organelles which can be retracted and 

 formed anew, depending upon activities of the organism. Four major 

 types may be distinguished — lobopodia, filopodia, myxopodia, and 

 axopodia. 



Lobopodia, which have relatively dense outer layers and more fluid 

 inner zones, are relatively broad pseudopodia with rounded tips. Short 

 or slender lobopodia may be hyaline, but larger ones usually show a clear 

 ectoplasm enclosing a granular endoplasm. Lobopodia are characteristic 

 of amoebae, certain flagellates, and certain testate rhizopods (Fig. 1. 5, F). 



Filopodia are slender hyaline pseudopodia which taper from base to 

 pointed tip and also tend to branch and anastomose. In addition, filo- 

 podia may fuse locally to produce thin webs of cytoplasm. The absence 

 of circulating granules helps to distinguish filopodial from myxopodial 

 nets. 



Myxopodia (rhizopodia, or reticulopodia), characteristic of the Foram- 

 iniferida, are filamentous structures (Fig. 1. 6, A) which branch and 

 anastomose into complex networks often covering a wide area. Such nets 

 are efficient food-traps and are fairly effective locomotor organelles. In 

 addition, the digestive activities of myxopodia are usually marked in 

 Foraminiferida (Chapter V). The comparatively dense inner zone of the 

 myxopodium has been considered fibrillar in structure (198). The fluidity 

 of the outer layer is indicated by the active circulation of cytoplasmic 

 granules, as illustrated by Elphidium (Polystomella) crispum (103). 



Axopodia (Fig. 1. 6, B) tend to radiate singly from the surface of more 

 or less spherical organisms (Heliozoida, Radiolarida). The axial filament 

 of a typical axopodium has been described as a fibrillar tube enclosing a 



