CHAPTER XXVI 



RESEARCH PROBLEMS IN THE OPALINID^ 



By 



Maynard M. Metcalf 

 The Johns Hopkins University 



CILIATA 

 PROCILIA TA 

 OPALINID^ 



PROTOOPALiNiN^, binuclcated 



Protoo patina (nine subgeneric groups, about thirty-six described 

 species) 



Zelleriella (about thirty-eight described species) 

 OPALiNiN^, muhinucleated 



Cepedea (six subgeneric groups, about thirty described species) 



Opalina (two subgeneric groups, about thirty-eight described species) 



EUCILIATA 



The opalinid^ are not only of interest in themselves, but are of 

 peculiar interest because of the light they throw upon the origin of 

 the EUCILIATA, whose remarkable nuclear condition (one transient 

 nucleus, large, metabolic in function; and usually one permanent 

 nucleus, small, carrying the genes functional in inheritance) is 

 unique among living things. In structure the opalinid^ are inter- 

 mediate between flagellata and euciliata. 



I. MORPHOLOGY 



1 ) The pellicle and its ridges have not been adequately studied, 

 discrepant accounts having been given. 



2) 'Alveolar structure" was first described by ButschH from 

 Opalina, but this foam was not truly alveolar as described. Students 



^For a description of the structure and life-history of an opalinid, Pro- 

 toopalina intestinalis, see Metcalf, 1923, introduction. For a bibliography of 

 the OPALINID.E and a brief, critical review of the literature see Metcalf, 1909 

 (literature previous to 1908) and 1923 (subsequent literature through 1922). 



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