448 University of California Publications mi Zoology [Vol. 20 



INTRODUCTORY AND HISTORICAL 



Early in the spring of 1921 there appeared in great abundance, 

 in one of my aquaria, a small euglenoid flagellate, which has been 

 identified as Menoidium incurvum (Fres.) Klebs. This form was 

 first described by Fresenius (1858) as Rhahdomonas incurva. Stein 

 (1878) later found this species but considered it to be merely a 

 "jugendliehe Form" of his Astasia proteus {=Distigma proteus 

 Ehrb.). Seligo (1885) found, in the same culture, Rhabdomonas 

 incurva, Astasiopsis distorta Duj., and a so-called intermediate form. 

 Influenced by the earlier statement of Stein he concluded that, since 

 he found no division stages of his Rhabdomonas, this flagellate was a 

 stage in the life history of Astasiopsis. However, both Biitschli 

 (1883-1889) and Klebs (1883) later recognized Rhabdomonas incurva 

 as a distinct species; finally, Klebs (1892), on the basis of the resem- 

 blance between Menoidium pellucidum Perty and Rhabdomonas in- 

 curva Fresenius, included the latter in the genus Menoidium Perty : 



Ich mochte jetzt die Gattung weiter fassen als Perty, Stein unci ieh selbst es 

 friiher gethan haben, indem ieh die Ehabdomonas incurva hinzuziehe, da in der 

 That der Untersehied zwisehen dieser Art und Menoidium pellucidum viel geringer 

 ist als derjenige zwisehen einzelnen Euglena-res]). Phacus- Arieji. Die Gattung 

 Menoidium unisehliesst dann die starren, eingeisseligen, ^«<asta-ahnliehen Flagel- 

 laten. 



The species Menoidium incurvum is described by Klebs (1892) 



as follows: . 



Kiirper cylindriseh, an beiden Enden abgemndet, meist etwas gekriimmt. 

 Plasmamembran niit weit von einander stehenden Langsstreifen versehen. Lange, 

 16-21;i; Breite, 7-8^. 



Klebs 's description coincides with my own observations, with the 

 addition that these flagellates always contain a number of nearly 

 colorless paramylum bodies, clumped together usually at the anterior 

 end, although sometimes present at both ends (pi. 40, figs. 1 and 6). 

 Since nuclear division and binary fission have not heretofore been 

 described in this flagellate, I shall attempt to discuss these processes 

 in this paper. 



I am indebted to Dr. Robert C. Rhodes of Emory University, at 

 whose suggestion this investigation was iindertaken, for his advice 

 and assistance ; also, I wish to thank Professor Charles A. Kofoid, 

 in whose laboratory this work was completed, for his invaluable 

 criticisms and suggestions. 



