Phylum Protozoa 353 



The first three classes are somewhat related and are grouped 

 together as the subph(ylum Plasmodromd. The latter two classes are 

 grouped as the subphylum Ciliophora. These relationships are at best 

 rather superficial ones, for this phylum is actually composed of a large 

 number of very diverse forms. Their only truly common character 

 is that all are one-celled or acellular organisms. They have evolved in 

 all ways and fit successfully into all types of habitats. 



THE CLASS FLAGELLATA 



Perhaps the most diverse class of this varied phylum is the Flagel- 

 lata or Mastigophora. The only character which these many forms 

 have in common is the possession of one or more flagella for loco- 

 motion. So versatile is this group in regard to form, nutrition, repro- 

 duction, and colony formation that it is usually considered to be the 

 type from which arose the other protozoan classes, the plant kingdom, 

 and the remainder of the animal kingdom. This is only a speculation, 

 but is based upon careful observation and consideration. 



Some members of this group which contain chlorophyll are holo- 

 phytic in their nutrition and are grouped as the subclass Phyfomastigina. 

 Those forms which are either holozoic or saprozoic are grouped as the 

 subclass Zoomastigina. As some members of this class are able to be 

 either holophytic or saprozoic depending upon conditions in the en- 

 vironment, this diflference is not always so distinct as the classification 

 would imply. 



Euglena. — The features of the subclass Phytomastigina, may be 

 illustrated by an animal commonly studied in the laboratory, Euglena. 

 Euglena (Fig. Ill) is a spindle-shaped animal, somewhat more pointed 

 at the posterior end. The body is enclosed in a thin membrane, the 

 pellicle, which has many tross striations. From the anterior end ex- 

 tends a long whiplike flagellum which is capable of a lashing motion 

 for propelling the animal forward as well as rotating it on its axis. 

 The flagellum is formed of a delicate protoplasmic sheath surrounding 

 an axial filament and is attached basally to a structure known as 

 the basal body or blepharoplast. This latter appears to control the 

 movements of the flagellum. At the anterior end of the organism is 

 an opening, the mouth or cytostome, through which the flagellum passes. 

 Behind the mouth is a short cytopharynx which leads into a spherical 

 reservoir. The blepharoplast is located to one side of the cytopharynx. 



