THE MASTIGOPHORA 155 



THE unicellular organisms which are associated in the class Mastigo- 

 phora or Flagellata in the wide sense, comprise a very heterogeneous 

 assemblage of forms, having in common the possession of certain 

 characteristic traits of organisation (a single nucleus, one or more 

 contractile vacnoles, one or more flagella), and further united together 

 phyletically by the occurrence of transitional or annectant types. 



Our knowledge of the group dates back to the time of Anton 

 Leeuwenhoek, at the beginning of the eighteenth century, while 

 the foundation of the modern system may be safely attributed to 

 the labours of Christian Gottfried Ehrenberg during the early part 

 of last century (1830-1838). 



From the most general point of view the peculiar biological 

 interest of the Mastigophora rests upon the fact that, in this more 

 than in any other class of Protista, the formal distinctions which 

 are commonly drawn between the animal and vegetable kingdoms 

 vanish. It was formerly a question whether such and such an 

 order of Mastigophora should be reckoned among the unicellular 

 Algae or among the Protozoa, but this controversy is now practi- 

 cally over, and biological disquisitions upon the group are equally 

 at home and equally necessary in zoological and botanical treatises 

 and journals. 



When an organism possesses a green colour, due to the presence 

 of chloroplasts stained with chlorophyll, has a cell-wall that gives 

 the chemical reactions of cellulose, and is devoid of a mouth for the 

 ingestion of solid food, it is usually regarded as a plant. When, 

 on the other hand, an organism bears no chlorophyll, has no cell- 

 wall, or has a cell-wall that does not give the cellulose reaction, 

 and possesses a mouth for the ingestion of solid food, it is usually 

 regarded as an animal. 



If it were possible to divide the Mastigophora into two 

 divisions, one containing all those forms provided with a mouth 

 and devoid of chlorophyll and a cellulose cell-wall ; and the other 

 containing all those forms without a mouth, bearing chlorophyll and 

 surrounded by a cellulose cell-wall, then the former division could 

 be assigned to the animal kingdom and the latter to the vegetable 

 kingdom. Such a division would, however, be thoroughly un- 

 scientific and unnatural. It could only be made by deliberately 

 ignoring obvious genetic relationships. Moreover, such a division 

 would leave out of account a number of organisms particularly 

 Monadidea which fail to fulfil all the conditions for their admission 

 into either of the divisions. 



It is not by the study of any one stage of the life-history of 

 these organisms that it is possible to arrive at any clear conception 

 of the best distinction that can be drawn between the animal and 

 vegetable kingdoms. 



The study of the whole life -history of some of the lower 



