THE OPALIXID CILIATE IXFUSORIAXS. 259 



Although some phases of the nuclear phenomena in Opalinidae 

 are still obscure, especially in their interpretation, enough is known 

 to indicate that the structure and behavior of the nuclei in this group 

 are more archaic than in Euciliata. There will be further reference 

 to this point in connection with the discussion of the relationships 

 of the Opalinidae to other groups (section 6). 



The nuclei of most species of Protoopalina and of all known species 

 of Zelleriella are remarkably favorable for study -^ because of their 

 large size and clearly seen chromosomes. In the Zelleiiellas the 

 flattening of the body is a further aid to study, for in some forms 

 with large nuclei the nuclei occupy the whole thickness of the en- 

 dosarc, being separated from the observer only by the ectosarc. The 

 animals are already in effect microscopic sections. 



5. RELATIONSHirS AMONG THE OPALINIDAE, AND THE CRITERIA OF 



SPECIES. 



Of the four genera of Opalinidae, Protoopalina seems the more 

 archaic. Its cylindrical form is like that of the less modified of the 

 Flagellata. Zelleriella may well have been derived from Proto- 

 opalina by mere flattening of the body. It seems to have arisen after 

 the binucleated condition had been established, for we know no 

 uninucleated species of Zelleriella. There are no indications of a 

 polyphyletic origin of the Zelleriellas. In the subfamih^ Opalininae 

 we may again regard in general the cylindrical Cepedeas as the more 

 primitive, and the uniformly flattened Opalinas as derivative. The 

 cylindrical forms are sufficiently distinct to justify placing them in 

 a separate genus, Cepedea. The uniformly flattened multinucleated 

 forms, the Opalinas, may perhaps have arisen along more than one 

 line, if we may trust form of body and histological appearance as a 

 guide to relationships in the Opaliniriae. There are at least two 

 series of species intergrading between the cylindrical condition, on 

 the one hand, and the uniformly flattened form on the other. The 

 indications are not decisive, but are sufficient to produce the impres- 

 sion of possible polyphyletic origin of the flattened forms. We do 

 not. however, have sufficient data for subdividing the flattened forms 

 into the several genera which, on the basis of polyphyletic origin, 

 would represent their true genetic relationship. Instead we must 

 continue the old genus Opali7ia, although possibly it is not a true 

 taxonomic entity, but merely a convenient term by which to refer 

 to the uniformly flattened multinucleated species. One intergrading 

 series between the Cepedeas and the Opalinas consists of the species 

 Cepedea magna (fig. 117, p. 152), O. ohovoidea (fig. 118, p. 153), and 



=« In the autumn of 1906 I'rof. Theodor Boveri suggested to me to study PrutooimUna 

 inteatinnJis and P. catidata, saying that they had the most Ix-autiful nuclei ho knew 

 junong the Protozoa. 



