SPECIAL MORPHOLOGY AND TAXONOMY OF SARCODINA 317 



Filopodia are homogeneous hxaline pseudopodia possessing in 

 many cases a remarkable elasticity and power of independent move- 

 ment. It is possible that these pseudopodia do not represent the 

 clear ectoplasm of the Amoeba type of pseudopodium, but possibly 

 are homologous with the stereoplasmatic part of a myxopodium, 

 or the highly modified representative of an axial filament. 



Lobopodia finally cannot be interpreted properly as motile organs. 

 They are characterized by nothing that can be homologized with 

 structural parts of other tx^Des of pseudopodia. They are depen- 

 dent upon the physical condition of the protoplasm from which 

 they are formed and are present in any t^pe of cell and in any type 

 of animal in which such physical conditions preA^ail. They are by 

 no means limited to the rhizopods amongst Protozoa but as shown 

 in the last chapter, are characteristic of many tv-pes of flagellates 

 as well, and they are formed by one t^•pe of cell or another in the 

 majority of higher animals. 



It is possible of course that the path of e\'olution has been exactly 

 the reverse of that outlined above and that progressive e\'olution 

 has resulted in the gradual differentiation of the more complex 

 types of pseudopodia until with Heliozoa we have a protot\']3e of 

 the jNIastigophora. Such an hypothesis makes it more difficult, 

 however, to accoiuit for such forms as the Bistadiidse or the flagel- 

 lated phases of different t.^'pes of Sarcodina. 



All type?, of reproduction are represented; simple division, budding 

 division, unequal division and multiple division (p. 20S) and the 

 life histories of different types are so variable that a common or 

 generalized account would be inadequate. In general it is legiti- 

 mate to say that a two-phase, metagenetic, life history is charac- 

 teristic although certainly not universal. Sexual processes are more 

 widely distributed throughout the sub-phylum than they are in 

 the JNIastigophora but here again, these cannot be described as 

 any common type. 



Encystment or resting stages are well known in fresh-water forms 

 of Sarcodina but are absent, or have not been described in connec- 

 tion with representatives of the two great groups of marine forms— 

 the Foraminifera and Radiolaria. 



Classification of the Sarcodina is fairly well established although 

 minor differences depending upon the individual judgment of rela- 

 tionship in special cases will be found. Division into main groups 

 is made on the basis of pseudopodia t\'pes while minor groups are 

 based upon special structural or functional peculiarities. Thus one 

 great group is characterized by the possession of ray-like pseudo- 

 podia with axial filaments and is given here the taxonomic value of 

 Class I, the Actinopoda, and these show the nearest approach to the 

 Holomastigidce amongst the flagellates. A second group— Class II 

 —includes forms with myxopodia, filopodia and lobopodia and is 



