248 BULLETIN 120, UNITED STATES NATIONAL MUSEUM. 



(rati arum-like forms), is emphasized by the fact that the slender 

 species are American, while the broad species occur in the eastern 

 hemisphere. These subgeneric <^roups are not sharply demarcated, 

 for some of the western hemisphere species have broad individuals 

 and some of the eastern hemisphere species show narrow forms. It 

 i§ not possible to be quite sure, in some few instances, to which sub- 

 generic group a species belongs. The American group has sent one 

 known representative into Asia and Europe, and the Opalinae latae 

 of eastern Asia have sent a few representatives into western America. 

 Further reference to these two groups of species of Opalina will be 

 ma^le in discussing geographical distribution (sec. 7). 



Only the four genera, Protoopalina^ ZellerieUa^ Cepedea^ and Opa- 

 lina^ are to be included in the family Opalinidae. Other genera of 

 astomatous Ciliata, which have often been included in the family, 

 are fundamentally different in nuclear character. The relationships 

 of the Opalinidae to one another will be discussed in section 5, and 

 the relationships of the family Opalinidae to other Protozoa will be 

 discussed in section 6. 



Subgeneric groups among the Opalinids are discussed in section 7 

 of this paper in connection with problems of distribution. In that 

 discussion we will find reason for believing that Protoopalina and 

 Cepedea are old genera and that Zdleridla and Opalina were more 

 recently evolved, and we will note that the two older genera are divis- 

 ible into subgeneric groups of species, while Zelleriella is a very com- 

 pact genus with no such subgeneric group, and that Opalina shows 

 only the division in Opalinae latae and Opalinae angustae. The two 

 older genera have developed a more divergent speciation. 



It is worthy of note that the tendency to become flattened has ap- 

 peared at least twice in the Opalinidae, each subfamily containing a 

 more archaic cylindrical genus from which has been derived a flat- 

 tened genus. As already noted, it is possible that flat Opalinas have 

 arisen twice or even oftener from Capedeas. 



4. THE NUCLEAR CONDITIONS IN THE OPALINIDAE. 



The Opalinidae show several features of nuclear structure and 

 behavior that are of much interest : 



1. Alone among the Ciliata, they have two or more nuclei which 

 are all alike, all other Ciliates having two nuclei differentiated to 

 form, one a metabolic meganucleus, the other a reproductive micro- 

 nucleus; temporarily in some Euciliates, the macron ucleus may be 

 divided into numerous portions and at certain stages in the life 

 cycle there may be several micronuclei. 



2. Each nucleus of the Opalinids has both metabolic and repro- 

 ductive chromatin fully developed and functional, the metabolic 



