434 BIOLOGY OF THE PROTOZOA 



While many of the Sarcodina are typically uninucleate it may be 

 safely stated that this is exceptional in the group as a whole for 

 the vast majority of Mycetozoa, Foraminifera and Radiolaria are 

 multinucleate. Nuclear dimorphism, however, does not occur and 

 the multinucleate condition is brought about by fusion of cells to 

 form plasmodia as in the Mycetozoa, or by repeated division of 

 nuclei without accompanying division of the cell as in the Fora- 

 minifera and Radiolaria. 



Contractile vacuoles are typical of fresh water forms and their 

 absence is equally typical of salt water and parasitic forms of 

 Sarcodina. When present they are invariably simple and burst 

 directly to the outside without reservoirs, canals or permanent 

 pores, and they furnish the best evidence for the view that contrac- 

 tile vacuoles here are primarily regulatory in a physical sense, 

 rather than excretory, in function. 



The most characteristic feature of the Sarcodina as a group is the 

 ability of the individual cell to throw out protoplasmic processes 

 called pseudopodia, and movements of translation or in food- 

 getting are brought about by the protoplasm in the formation of 

 these processes. It was this ability which led Dujardin in 1841 to 

 distinguish these types as les rhizopodes from lesflagelles and les fibre*. 



Pseudopodia, however, cannot be described by any one definition. 

 The most casual student of the Protozoa will not fail to recognize a 

 difference between the pseudopodia of Amoeba proteus and those of 

 an Arcella or Difflugia, while the difference is even more marked 

 between these types and the pseudopodia of any foraminiferon, or 

 between these and any heliozoon. These differences are so pro- 

 nounced that modern students of the Sarcodina beginning with 

 Lang have distinguished no less than four types of pseudopodia 

 under the names of axopodia, myxopodia, filopodia and lobopodia, 

 and there is some evidence that these four types and in the order 

 given represent adaptations of a degenerative nature from an ances- 

 tral flagellum-like type of motile organ. 



Axopodia are homologous with the flagellum of Mastigophora 

 (p. 145). An axial filament extends from the endoplasm to the 

 tip of the pseudopodium. Many protozoologists, following Doflein, 

 regard this as essentially a supporting structure, but like the axial 

 filament of a flagellum in many cases it is derived from a kinetic 

 element in the endoplasm and as in the hypermastigote flagellates 

 the axial filaments in many forms form the astral rays of an amphi- 

 aster at division (c g., Dimorpha mutatis. Fig. 79, p. 148). In place of 

 the periplastic sheath of the flagellum an axopodium has an invest- 

 ing sheath of cortical plasm in which the protoplasmic granules 

 may be seen streaming back and forth. Many are elastic or mildly 

 vibratile and undoubtedly belong in the category of motile organs 

 since movement of the organism is dependent upon their activity. 



