316 BIOLOGY OF THE PROTOZOA 



form Plasmodia as in the INIycetozoa, or hy re])eated division of 

 nuclei without accompanyino; division of the cell as in the Foramin- 

 ifera and Kadiolaria. 



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 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 indi\idual cell to throw out protojilasmic processes 

 called pseudopodia. It was this ability which led Dujardin in 

 1841 to distinguish these types as les rhizojJodes from les fiagellees 

 and les ciliees. 



Pseudopodia, howe\'er, 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 ijwteus and those of 

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

 between these types and the i)seudopodia 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 t^T^es of pseudopodia 

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

 and there is some e\idence that these four types and in the order 

 gi\'en, 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. 140). An axial filament extends from the endoplasm to the 

 tip of the pseudopodium. Like the axial filament of a flagellum it 

 is deri\'ed 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 amphiaster at division. In place of the peri- 

 plastic sheath of the flagelhnn an axopodium has an investing sheath 

 of ect()])lasm in which the protoplasmic granules may be seen 

 streaming ])ack and forth. ]\Iany are elastic or mildly vibratile 

 and undoubtedly belong in the category of motile organs since 

 movement of the organism is dependent upon their activity. 



Myxopodia are so called because of the tendency to fuse or 

 anastomose when two come in contact. The investing sheath of 

 protoplasm is highly miscible and upon fusion of many pseudopodia 

 a mesh or network, peculiarly characteristic of the Foraminifera, is 

 formed. Li this type the axial filament f)f the axopodia is absent; 

 in its place there is a medidlary core of denser substance termed 

 stereoplasmatic axis by Doflein, and interpreted by some as a 

 reminiscence of an earlier axial filament. 



