612 PROCEEDINGS OF THE NATIONAL MUSEUM vol.87 



cleate, for example, Arcella, Giardia, the Euciliata. Some amoebae 

 are usually binucleate and some are multinucleate (Pelomyxa). Some 

 euciliates are multinucleate in certain phases of their life cycle. 



In the genus Protoopalina one fission (most species), two fissions 

 (P. quadrinucleata) , or three or more fissions (P. axonucleata) may be 

 suppressed ; in Zelleriella only one; in Cepedea and Opalina many. Zel- 

 leriella arose from some species of Protoopalina in which multinucle- 

 ation had not appeared. It arose in Patagonia or Antarctica where 

 those species of Protoopalina that are approaching multinucleation 

 (subgenus VIII) do not occur. These are eastern Asian or Malaysian 

 species. The geographical distribution of Zelleriella and its ancestors 

 thus agrees with the absence of any developed tendency toward mul- 

 tinucleation. 



We have spoken of a trend toward suppression of one or more 

 fissions. There are other trends in the family — a trend toward 

 flattening, which receives tv/o independent emphases, first, in the 

 formation of Zelleriella, second, in the formation of Opalina; a trend 

 toward elongation, which likewise received two independent (?) 

 emphases, in the elongated Cepedeas, and in the elongated Proto- 

 opalinas; and others, I believe that the whole evolution can profit- 

 ably be discussed from the standpoint of trends, their occurrence, their 

 origin, their growth, their waning, their disappearance,^^ their inde- 

 pendence, their interdependence (see Metcalf, 1927a). We find evi- 

 dence as to the part of the earth and the geologic time in which 

 appeared emphases upon certain trends: for example, emphasis upon 

 flattening appeared once in southeastern Asia or in Lemuria in the 

 Cretaceous period (Opalina), and once in Patagonia at some time 

 between the middle Cretaceous and the middle Miocene, probably 

 during the early Tertiary. 



In no group of organisms is there better chance to study the nature 

 of the organisms them^selves as expressed in their evolution, relatively 

 undisturbed by pressure of their environment. In the evolution of 

 forms that have left even the fullest fossil record, it is very difficult to 

 evaluate the environmental factors and the internal factors. The 

 relative importance of the two classes of factors may be very different 

 in various organisms. One should, therefore, be very cautious in 

 drawing general conclusions from one group and applying them to 

 another group. 



One feature of the evolution of the Opalinidae seems of rather 

 general application to internal parasites. The adaptations to para- 

 sitism, if they occur at all,^^ are likely to take place promptly, and the 

 subsequent evolution to be comparatively slow and slight because of 

 the removal of much of the pressure of the environment. We have 



» Suppression might be a better word than disappearance to use here. 



i>i Little structural adaptations to parasitism are observed in Batantidium and Nijctotherus, which live 

 with the opalinids in the recta of Anura. 



