216 



THE PROTOZOA 



between swarm-spores, either amcebulse or flagellulse. The microg- 

 amy is isogamous or slightly anisogamous ; macrogamy, as in other 

 cases, is perfectly isogamous. Microgamy occurs, as has been seen 

 (p. 148), in Arcella in addition to chromidiogamy ; and, according 

 to a recent note of Zuelzer (86, p. 191, footnote), syngamy between 

 free swarm-spores occurs in Difflugia also. 



As regards the life-cycle of the Sarcodina, there remains still so 

 much to be discovered that to generalize is both difficult and 

 dangerous. Even in the commonest forms, such as Amoeba proteua, 

 the complete life-cycle has not been yet worked out. In some 



FIG. 85. Changes in the form of an amoeba under the influence of differences 

 in the [surrounding medium. A 0, In its natural medium (water) : A, 

 contracted ; B, beginning to throw out pseudopodia ; C, Umax-form. D F, 

 Forms assumed after addition of potash-solution : I), contracted, beginning 

 to throw out pseudopodia ; E, F, radiosa-iorr&s. After Verworn. 



cases the life-cycle appears to be of comparatively simple type, and 

 the species is monomorphic or nearly so, as in Actinosphcerium ; 

 in other cases there is a well-marked alternation of generations, 

 with dimorphism in the adult condition, as in Trichosphosrium 

 (p. 182), the Foraminifera, etc. 



Classification. The Sarcodina are subdivided into a number of 

 orders, the distinctions between which are based principally on the 

 characters of the pseudopodia and of the skeleton, when present ; 

 in more highly differentiated forms, such as Radiolaria, the internal 

 structure of the body is also taken into account. In the primitive 



