PROTOZOA AND OTHER ANIMALS 929 



If it could be shown that there is resistance to cross infection, such 

 that flagellates introduced experimentally from a natural host species 

 into another one would not survive, this thesis would of course be sup- 

 ported. The writer (1937), however, stated that there seems to be no 

 resistance to cross infection, basing this opinion on experiments by Light 

 and Sanford (1927, 1928) and Cleveland et al. (1934). No experi- 

 ments yet reported, however, have been continued long enough to war- 

 rant any definite conclusion. Unpublished experiments by Dropkin, 

 furthermore, showed that Protozoa of Retkulitermes flav/pes, Kalo- 

 termes schivartzi, and K. jouteli could not establish a physiological rela- 

 tionship with Zootermopsis sufficient to permit survival of the termite 

 for more than fifty days in the absence of the normal faunule. 



Although there has been evolutionary development of the flagellates 

 within termites of groups that exist today, many of the types doubtless 

 go back to ancestral insects. The genus Trichonympha, being found in 

 both termites and Cryptocercus, may be supposed to have passed into both 

 these insects from ancestral protoblattids (Kirby, 1937). The distribu- 

 tion of Trichonympha in termites alone would indicate its antiquity and 

 stability (Kirby, 1932b) . The existence of representatives of other hyper- 

 mastigote groups in Cryptocercus indicates the very ancient differentia- 

 tion of those flagellate types. By loss of members of the faunules here 

 and there, together with continued but less drastic evolutionary changes, 

 the present composition of the faunules may have originated. The flagel- 

 lates were probably present in ancestors of Termitidae, but were, in the 

 course of differentiation of those insects, dropped out. The origin of 

 the amoebae needs to be explained; possibly they were acquired later. 



Adaptive Host Relationships in Morphology 

 AND Life History 



GENERAL CONSIDERATIONS 



Structural modifications in animals that live in association with hosts 

 take two general forms. There are morphological changes in direct 

 adaptation to the requirements of the habitat; and there are changes un- 

 related directly to that habitat, but made possible by various factors in 

 it. In the former group, among Protozoa, is the development of or- 

 ganelles of fixation, though this development is not restricted to Protozoa 

 that live in close relationship with other animals. Special adaptations 



