Protozoa and Protophyta. 63 



Chapter II. section v. For instance, flagellate and amoe- 

 boid motion are possible only in a liquid (water). If that 

 were absent the protoplasm would dry up and shrivel, or the 

 surface would become covered by a skin ; in other words, 

 encystation would take place. Those forms, therefore, 

 which exhibit the complete life-history of Protomyxa are 

 necessarily such as live in liquid media. It will be easily seen 

 from this that the varying length in the duration of the seve- 

 ral stages can be accounted for by the habits of the form in 

 question, whether living all its life in a fluid, or only a part 

 of its existence in such a medium. 



In the cases hitherto discussed we have dealt with 

 organisms composed of one cell only. To such organisms 

 the term unicellular has been applied. Now, manifestly, 

 the term * unicellular organism ' might be applied to or- 

 ganisms which were referable either to the plant or to the 

 animal world. We have already seen (p. 38) that the green 

 colouring matter chlorophyll is essentially characteristic of 

 the plant as distinguished from the animal, and we have 

 just learnt that a cellular investment is an additional charac 

 teristic of a vegetal cell. We are able, therefore, roughly, to 

 divide unicellular organisms into vegetal unicellular organ- 

 isms and animal unicellular organisms. To the former class 

 we give the name of Protophyta, or ' primitive plants ' ; while 

 to the latter we give the name of Protozoa, or ' primitive 

 animals.' There are, however, many organisms which, as 

 has been already indicated, cannot be said to belong 

 directly to either group, e.g. Protomyxa itself. These are 

 grouped along with the Protophyta and Protozoa in one 

 general collection, to which the term Protista, or ' primitive 

 living things,' is applied. A protist is therefore a unicellular 

 organism, whether truly vegetal or animal or doubtful in its 

 affinities ; in other words, an extremely simple living thing. 

 A protozoon is a unicellular organism, generally without 

 chlorophyll, and without a cellulose investment ; in other 

 words, an extremely simple animal. A protophyte is a 



V 



^, n 



