24 BULLETIN 120, UNITED STATES NATIONAL MUSEUM. 



of the life-history must be followed again to determine exactly 

 these relations. 



In the definitive gametes we have uninucleated individuals, each 

 nucleus containing four rather small chromosomes and some achro- 

 matic granules. The fusion of these nuclei gives nuclei with eight 

 chromosomes (fig. 8, c). An interesting complication is introduced 

 in the fact that fusion of the gametes may occur before the macro- 

 gamete has completed its final division and while it still has two 

 nuclei. But for discussion of this point I would refer to a former 

 paper (Metcalf, 1909). 



It is evident from this life history, as well as from the nature of the 

 ordinary vegetative fissions, that Protoopalina intestinalis is not a 

 truly binucleated form. It is normally uninucleate at the time of 

 sexual union, and it is uninucleate immediately after ordinary vegeta- 

 tive fission, before the division of the nucleus in the daughter cell has 

 been completed, thus reestablishing the binucleate condition. The 

 two daughter cells, in this species, usually separate while their nuclei 

 are in a late anaphase, or often a telophase condition. These 

 phenomena will be found significant in the discussion of speciation in 

 the Protoopalinas. 



There is little needs here be said of the biology of the Opalinids. 

 They seem to live as commensals with their hosts, rather than as true 

 parasites. They apparently feed upon the wastes in the rectum, and 

 so far as can be determined are not injurious to the animal which 

 furnishes them their secluded and advantageous habitat. It is, of 

 course, altogether improbable that the Opalinidae reached their 

 present very considerable diversity before they acquired their 

 parasitic habit. Were this the case, there would probably still be 

 free-living forms showing undoubted resemblance to the parasitic 

 members of the group. Their divergence must have taken place under 

 conditions of parasitism. But the secluded habitat in the recta of 

 their hosts, and the abundance of predigeated food, make conditions 

 which do not emphasize the struggle for existence, and their environ- 

 ment does not present sufficient diversity to encourage diverse adapta- 

 tions to divergent conditions of environment. Free-living Flagellates 

 and Ciliates have structural features adapted to seeking and ingesting 

 solid food. The Opalinids have lost all trace of structures associated 

 with ingestion, but retain the locomotor organs. The further modi- 

 fications in form and nuclear relations, which are so marked among 

 the members of the family, were almost certainly developed during 

 their life as parasites. Since the uniformity and favorable character 

 of their life conditions during parasitism do not encourage diversifica- 

 tion in structure through natural selection, it seems evident that such 

 diversification as has occurred has been acquired through divergent 

 orthogenesis, if we may use this Hibernianism. The causes of the 



