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Experientia 43 (1987), Birkh~iuser Verlag, CH-4010 Basel/Switzerland 1229 avoided phagocytosis by normal murine macrophages. Organisms of this latter subpopulation remained in the supernatant of cultures of L929 cells or macrophages for prolonged periods of time and when inoculated into mice produced infections with parasitemias and survival times that were significantly lower and longer than those noted in mice inoculated with preparations containing trypanosomes of both subpopulations. This observation is in agreement with the report by Howells and ChiarP 7 who showed that mice inoculated with trypanosomes recovered from the blood of other mice inoculated with the organisms two days before had longer prepatent parasitemias and increased survival times than controls. In addition, when trypanosomes of the 'non entering' population were added to acellular medium and cultured at 28 ~ they immediately started multiplying and a slight increase in the number of organisms could be noted as early as 36 h of culture. In contrast, when a control preparation containing organisms of both populations was similarly cultured a decrease in the number of organisms was noted at 36 h of culture. The decrease in the number of organisms was not statistically significant but may have indicated death of those trypanosomes that were unable to differentiate into epimastigotes and multiply as such, possibly because they had been biologically programed to invade cells and maintain the infection in the mammalian host. So far, attempts to identify a particular subpopulation of parasites responsible for maintenance of the infection in mammalian hosts have not been successfull. The clonal approach has revealed the existence of differences among individual parasites in regard to invasiveness and pathogenicity 5. Comparison of trypanosomes obtained from insect vectors, from bloodstream of mammalian hosts, from cell cultures or from axenic cultures have also demonstrated differences in invasiveness and in lethality TM 17 19. Long term maintenance of parasite strains in cultures or in laboratory mammalian hosts selects organisms with the highest ability to adapt to the culture conditions or to the experimental host but do not change the genetic make-up of individual organisms 2~ 21. Heterogeneity, however, has to be intrinsic to each organism and its expression genetically regulated since differences present in isolates kept in the laboratory for long periods of time were also demonstrated in parasites derived from a single clone.At the moment, lectins are being used in our laboratory to examine the changes that occur on the ceil membrane surface of trypanosomes following their retrieval from mammalian hosts and culturing until their final differentiation into epimastigotes.Summary. During suction feeding, the primitive aquatic salamanders Cryptobranchus alleganiensis and Andrias japonicus frequently display asymmetric movements of the lower jaw and hyoid apparatus that have some similarity to working and balancing side kinematics of mammalian ma...

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