356 BULLETl-\ iliU, UNITED STATES NATIONAL MUSEUM. 



America, from some broad Opalina, and probably in some species of 

 the Hylidae whicli adopted and proceeded to modify this broad 

 Opalina; that this could not have occurred earlier than the Miocene 

 or more probably the Pliocene, for Hylids were not in Central or 

 North America before the Miocene or Pliocene ; that this transforma- 

 tion of broad Opalinas into narrow Opalinas occurred before the 

 earliest glacial period, since a narrow Opalina Avas carried by a Hyla 

 to Euro-Asia by way of Alaska, a migration improbable after the first 

 glaciation of the Quaternary; that the Opalinae angustae are the 

 most recently evolved of the groups of Opalinidae, they, and prob- 

 ably also their ancestors the Opalinae latae, being even more modern 

 than the Zelleriellas. 



One of the most interesting results of our study of the Opalinidae 

 is seen in the indication that ancient subgeneric groups, some of them 

 probably of Triassic origin, are still extant. For example, a group 

 of species of the genus Frotoopalina (including P. acuta and P. 

 papuensis from Australia, P. diplocarya from South America, and 

 P. xenopodos from tropical Africa), archaic in character, are found 

 to-day so distributed in the three great southern continents as to be 

 an indication of their origin in the Triassic period, when these three 

 land areas were united to form the continent Equatoria. In this and 

 in other instances, especially in the genus ZelleHella^ we find very 

 closely similar species present in widely separated areas — group 1 of 

 the genus Protoopalinu in Africa, South America, Australia, and 

 Papua ; Protoopalina^ group 4, in Europe and Australia ; Protoopa- 

 lina, group 8, in Africa and South America ; almost identical species 

 of Zelleriella in South America and Australia ; Cepedea^ division 2, 

 in Euro- Asia and South America ; Gepedea, division 4, in Africa and 

 Florida; Cepedea, division 5, in Asia and tropical America; Gepeded^ 

 division 6, in the Seychelles, Malaysia, Asia, and in North America; 

 and Opalinae angustae in North America and Euro-Asia. 



It would be difficult to parallel these conditions from many groups 

 of free-living animals. Apparently it is chiefly the parasitic life, in 

 which the animals are protected and fed and are without much variety 

 of environing conditions to make divergent adaptation advanta- 

 geous, that has allowed the Opalinids to persist through a number of 

 geologic periods with so little modification in numerous instances. 

 Of course there was an early and very considerable modification to 

 fit these organisms in the first place to their habitat within their 

 hosts. Their loss of a "mouth" was the most noticeable structural 

 feature in this early modification. There has been a good deal of 

 tlivergence since the parasitic habit was acquired, so that now we may 

 recognize two subfamilies, four genera, and certain subgeneric groups 

 of si.pfu's I'>nt among the Opalinids we find to-day numerous species 



