PHYLUM PROTOZOA 



39 



The nucleus is frequently single, though binucleate or multi- 

 nucleate individuals are common in some orders. The nuclei in 

 these latter are essentially similar rather than manifesting the 

 functional differentiation which is characteristic of the macro- 

 and micronuclei of the ciliates. 





B 



^A««»».i.x.;^^^,. 







^4.J^ 



Fig. 24.— Types of pseudopodia. ^-1, eruptive types of lobopodium; B, 

 myxopodia of Foraminifera; C, axopodium of Heliozoa. {Redrawn from Calkins). 



Binary and multiple fission and budding arc all represented in 

 the types of reproduction among the Sarcodina, and some forms 

 pass through a definite life cycle involving conjugation (Arcella). 

 The relatively simple organization of the Rhizopoda has led 

 many students to assume that only a simple cleavage of the cell 

 is involved in the reproductive process. It is probable that 

 simple fission is not the sole phenomenon in the ontogeny of many 

 forms. Encystment, so common a phenomenon in fresh-water 

 and parasitic Sarcodina, has not been observed in Foraminifera 

 and Radiolaria, the two chief groups of marine forms. 



