CLASSIFICATION 67 



the record is very incomplete, especially as regards the very perish- 

 able structures of the lower plants, and we can never expect to have 

 much light thrown on the origin of these lower plant-types, from a 

 study of fossils. 



Classification 



The vegetable kingdom may be divided into a number of primary 

 groups, " Subkingdoms," or " Branches," as to whose limits there is 

 a good deal of difference of opinion. We shall assume here five of 

 these subkingdoms, viz. Schizophyta, Algse, Fungi, Archegoniatae, 

 Spermatophyta. Besides these there are two groups of organisms, 

 sometimes included among plants, the Myxomycetes (Mycetozoa) 

 and the Flagellata, both of which show unmistakable animal affini- 

 ties as well. 



Each subkingdom is divided into classes, these into orders, fami- 

 lies, genera, and species, which are sometimes still farther subdivided. 



THE SIMPLEST ORGANISMS 



Many of the lowest organisms known are so simple in structure as 

 to make it impossible to decide positively whether their affinities are 

 with plants or animals. They are simply undifferentiated living be- 

 ings, such as we may reasonably infer existed before there were any 

 true plants or animals. 



Protista. To these lowest forms of life Haeckel gave the name 

 " Protista," and assumed that some of them consisted of quite undif- 

 ferentiated protoplasm. The more perfect methods of investiga- 

 tion now in use have demonstrated that it is exceedingly doubtful 

 whether any organisms of such extreme simplicity really exist, and 

 most of the Protista have been relegated to one or the other of the 

 two great organic kingdoms. Nevertheless, there are two groups of 

 organisms, the Flagellata and the Myxomycetes or Mycetozoa, which 

 seem to lie on the border line between plants and animals. 



FLAGELLATA 



The Flagellata (Fig. 48) are unicellular organisms, which are provided with 

 one or two (occasionally more) flagella or cilia, by means of which they are able 

 to move rapidly in the water. The cell may be quite naked, or there may be a 

 more or less marked membrane, which very rarely, however, is composed of 

 cellulose. The cell contains a single nucleus, and sometimes chrornatophores, 

 which may be either green (Euglena) or brown (Hydrurus). The forms which 

 possess chromatophores are able to assimilate carbon-dioxide, like normal plants, 

 but those which are destitute of these feed upon organic matter. Some of the 

 more highly organized forms possess a mouth, so that they can ingest solid food, 

 which in the lower forms may be taken in at any part of the protoplast. 



