1919] Rhodes: Binary Fission in Collodictyon triciliatum Carter 239 



7. In mitosis the nuclear membrane is persistent. 



8. There is formed a perfect spindle of achromatic mantle-fibers 

 with at least seven, probably eight, chromosomes and an equatorial 

 plate. 



9. Life cycle so far discovered is simple, binary fission by longi- 

 tudinal division being the only method of reproduction known. 



Collodictyon is typically an animal of simple organization. Its 

 phylogenetic stem grows out of the unknown past. It is very close 

 to the stem from which the Rhizopoda and Mastigophora branch, 

 having much in common with each. That Collodictyon is one of the 

 simplest and most primitive of the Polymastigina, there can be no 

 doubt. With the free living members of the genus Tetramitus (not 

 accepting T. ckilomonas as such) it finds its nearest relatives. Costia, 

 Tetramitus (saprophytic and parasitic), Trichomastix, Polymastix, 

 and Trichomonas are derivatives of either the Collodictyon type or 

 free living Tetramitus. Thus knowing possibly the simplest Tetra- 

 mitidae, I am prone to regard this group as not so complex in its 

 entirety as France (1899) would have us believe when he wished 

 Collodictyon related to the Monadaceae rather than the "complex 

 Tetramitaceae. " Much of this complexity may be interpreted as the 

 result of morphological changes resulting directly or indirectly from 

 parasitism. 



In Collodictyon the two basal granules with very faint rhizoplasts 

 connected with the microkaryosome are not necessarily so highly differ- 

 entiated, when it may be regarded as simply a slight advance over 

 Cercomonas (Wenyon, 1910, text fig. 18), due to the multiplication of 

 flagella, or a doubling of the flagella of a biflagellate type. Collodictyon 

 is a primitive polymastigote. 



PARASITISM AND SYMBIOSIS 



For some three months, January to March, 1916, the majority of 

 all individuals of Collodictyon were filled with algae, which were 

 identified as Chlorella vulgaris. Only at the period of longitudinal 

 division and in moribund stages would they become free from these 

 inclusions. The algae were arranged peripherally, just beneath the 

 surface and it looked at times as though Collodictyon were hollow. 

 Often when under observation these algae could be seen to form small 

 nodules and pop out of the pellicle. These were seldom surrounded by 

 a hyaline area indicating that they were being used for food, though 

 at periods nearly all were so absorbed. 



