156 THE MASTIGOPHORA 



animals and plants, however, suggests a line of distinction which is 

 perhaps more in accordance with a natural system of classification. 



In the life -history of Ulothrix, one of the Ulotrichaceae, an 

 example of an organism that is universally regarded as a plant, we 

 find two forms of cells. There are the cells of the filamentous 

 thallus, protected by a cell-wall, containing chlorophyll, and, under 

 favourable conditions, growing and increasing in number by fission ; 

 and there are the cells provided with two or four flagella that 

 escape from their cellulose investments and eventually conjugate to 

 form a motionless zygospore. 



If we compare this with the life-history of such a form as 

 Mastigella, one of the Mastigophora that is universally regarded as 

 an animal, we find that during the phase of life when growth and 

 repeated multiplication by fission occurs the organism is actively 

 moving about by means of its flagellum or its pseudopodia, and that 

 the gametes that it gives rise to are also active and flagellate. 

 Any period in the life-history of Mastigella when active movements 

 cease is not, as in the case of Ulothrix, a period of vegetative 

 growth. 



If we regard, then, as marks of distinction between an animal 

 and a plant (1) that the stage of growth and reproduction of 

 somatic cells by fission is marked by a period of active mobility in 

 the former, and of stability in the latter ; and (2) that the flagellate 

 cells of the latter do not grow and divide by fission, but conjugate 

 and give rise immediately to a sedentary zygospore, whereas in the 

 former the flagellate cells may grow and divide by fission, we 

 represent a consideration which has had considerable weight in 

 determining the action of zoologists in including the Mastigophora 

 in the animal kingdom. But the boundary thus drawn, even if it 

 is the best that can be drawn, is itself subject to some exceptions. 



In some of the Chlamydomonadina we find, for example, that 

 flagellate individuals similar in general characters to the gametes 

 form a gelatinous investment, withdraw their flagella, grow and 

 divide repeatedly by fission. It is difficult to distinguish this phase 

 of life (the " palmella-stage," as it is called) from a true plant under 

 the terms of our definition. The close relation of the Chlamy- 

 domonadina to the Chromomonadina, however, is so clear that to 

 separate them by placing one order in the vegetable kingdom and 

 the other in the animal kingdom on this ground alone would be 

 absurd. 



The life-history of the Chlamydomonadina seems to support 

 very strongly the view that some of the families of the lower Algae 

 have sprung from a flagellate ancestry, but it does not justify the 

 assumption that the vegetable kingdom as a whole owes its origin 

 to the class Mastigophora. 1 



1 See Blackman and Tansley (2), and West (22, pp. 32 et seq.). 



