RESEARCH PROBLEMS IN THE OPALINID^ 239 



tribution. This suggests a series of infection experiments, including 

 cross-infections, with a view to ascertaining the influences which 

 affect host-parasite comity. In such studies two features, especially, 

 should be in mind: i) The influences which induce rapid, repeated 

 division and encystment of the opalinids just before the egg-laying 

 period of the host (these processes are, of course, essential to the 

 persistence of the opalinids from year to year) ; 2) the relation 

 of the metamorphosis of the host to the welfare of the parasite. 



As to the first point, one should remember that, in the tadpoles, 

 cysts continue forming for weeks and that the same is true in the 

 intestine of Box hoops, the Mediterranean fish which harbors 

 ProtoopaUna safurnalis. As to the second point, it is interesting to 

 note that, at least for some anuran species, such as Rana cafes- 

 beiana and R. clainitaus, the adults are uninfected, though the 

 tadpoles are practically always richly infected. It should further 

 be noted that the percentage of infected individuals among adult 

 AXURA, of some species at least, is less, for many species much 

 less, than among their tadpoles. Hyla versicolor is a conspicuous 

 example. A review of the several species of axura as to the rela- 

 tive incidence of opalinids in larvae and in adults might well accom- 

 pany any study of the effects of metamorphosis of the host upon 

 the infection. The change of diet in the axura from larva to adult 

 and the changes in the gut during metamorphosis are, perhaps, 

 important in this problem. 



IV. PHYSIOLOGY 



i) It would be of especial value if methods of culturing opali- 

 nids might be found. The animals under unfavorable conditions 

 show distortions of the macrochromatin bodies in the nucleus 

 C'macrochromosomes''), and they show these distortions so 

 quickly, that these should serve as a sensitive index to the favor- 

 able or unfavorable influence of conditions of many sorts in the 

 culture medium. The injurious effect of exposure to oxygen makes 

 the problem of culturing a difficult one and would complicate the 

 experimental treatment if successful culture methods were devised. 

 Anaerobic intestinal parasites have been successfully cultured and 

 much further study is sure to be given to the methods of such 

 culture. The opalinids should be included in these studies. 



2) Can opalinids in the host be diminished or destroyed by in- 

 troducing into the intestine oxygen or substances which will give 



