MORPHOLOGY AND TAXONOMY OF THE SARCODINA 435 



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. In this type the axial filament of the axopodia is absent; 

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

 stereoplasmatic axis by Doflein, and interpreted by some as a 

 reminiscence of an earlier axial filament. 



Filopodia are homogeneous hyaline 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 Ameba type of pseudopodium, but may be 

 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 types of pseudopodia. They are" depen- 

 dent upon the physical condition of the protoplasm from which 

 they are formed and are present in any type of cell and in any type 

 of animal in which such physical conditions prevail. They are by 

 no means limited to the rhizopods amongst Protozoa but, as shown 

 in Chapter XI, are characteristic of many types of flagellates as 

 well, and they are formed by one type of cell or another in the 

 majority of higher animals. 



It is possible of course that the path of evolution has been exactly 

 the reverse of that outlined above and that progressive evolution 

 has resulted in the gradual differentiation of the more complex 

 types of pseudopodia until with Heliozoa we have a prototype of 

 the Mastigophora. Such an hypothesis makes it more difficult, 

 however, to account for such forms as the Bistadiidae or the flagel- 

 lated phase of different types of Sarcodina. 



All types of reproduction are represented; simple division, budding 

 division, unequal division and multiple division (p. 209) 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 

 widely distributed throughout the sub-phylum, but here again these 

 cannot be described as of 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 



