26 RESEARCH IN PROTOZOOLOGY 



ciliates play in causing disease. Reynolds and Looper ( 1928) have 

 done this for Hydrainoeba hydroxcfia infecting fresh-water polyps. 

 Experiments similar to these might be employed in the study of 

 other host-parasite relations. 



Assumption of role as interyial parasites: Probably many in- 

 ternal parasites have passed through a stage of external parasitism 

 during their development of parasitic habits. Indeed some protozoa 

 may occur as both external and internal organisms. Notable among 

 these are Hydramwha hydroxcna (Entz, 1912) and Trichodinopsis 

 par ado xa Claparede and Lachmann (1858). Besides these, rep- 

 resentatives of the genus Trichodina, which under ordinary con- 

 ditions are ectoparasitic forms, have been reported from the cir- 

 cular canal of medusoids (Arnold and Boulenger, 1915), from 

 the urogenital system of amphibia (Rosseter, 1886) and from the 

 bladder of amphibia (Fulton, 1923). In light of these findings 

 it seems possible for some of the external protozoa to be suc- 

 cessfully transferred to internal habitats. Very little has been done 

 along this line. The author has unsuccessfully tried to infect in- 

 ternally Microstomum caudatum with Hydranurha hydroxena. 



Control measures: Most efforts in the direction of control meas- 

 ures directed against ectoparasitic protozoa involved better aera- 

 tion. Barthelemy (1926) and Stiles (1893) offer suggestions con^ 

 cerning the control of I chthyophthirius. 



Host specificity: The epizoic protozoa, as well as most of those 

 discussed under the heading ectoparasitic protozoa, usually attach 

 themselves to several different kinds of hosts. It is seldom that 

 host specificity does not extend to all members of a given genus, 

 or even family, when they live under similar conditions. The fol- 

 lowing papers might be helpful to one making a study of this sub- 

 ject: Craig (1897), Fiebiger (1909), Reynolds and Looper 

 (1928), and Wenrich (1924/?). Wenrich has made a statistical 

 study of the protozoa found on the external surface of other 

 animals. Beginning with Vorticclla he finds only 6.8% of the fifty- 

 nine species associated with other animals, in Rhabdostyla there 

 are 54.5% of associated species, in Opercularia 58.0%, in Epistylis 

 63.3%, and in Scyphidia yi.^%. 



