70 ZOOLOGY SECT. 



Systematic Position of the Example. 



Euglena viridis is one of several species of the genus Euglena, 

 and belongs to the family Euglenidce, sub-order Euylcnoidea, and 

 order Flagellata. 



The presence of an anterior flagellum and the absence of a 

 collar, transverse flagellum, or tentacle, indicate its position among 

 the Flagellata. It is placed among the Euglenoidea in virtue of 

 possessing a single flagellum and a small gullet into which 

 the reservoir opens. The genus Euglena is distinguished 

 by its centrally placed nucleus, green chromatophore, red stigma, 

 and euglenoid movements. E. viridis is separated from other 

 species of the genus by its spindle-shaped body with blunt ante- 

 rior and pointed posterior end, and by the flagellum being some- 

 what longer than the body. 



ORDER 1. FLAGELLATA. 



The cell -body is usually ovoid or flask-shaped (Fig. 52, 6, 7, 9, 

 &c.), but may be almost ^globular (1), or greatly elongated (3). 

 Anterior and posterior ends are always distinguishable, the flagella 

 being directed forwards in swimming, and, as a rule, dorsal and 

 ventral surfaces can be distinguished by the presence of a mouth 

 or by an additional flagellum on the ventral side. They are, 

 therefore, usually bilaterally symmetrical, or divisible into equal and 

 similar right and left halves by a vertical antero-posterior plane. 



Some of the lower forms have no distinct cuticle, and are able, 

 under certain circumstances, to assume an amoeboid form (2). 

 The curious genus Mastigamceba (4) has a permanently amoeboid 

 form, but possesses, in addition to pseudopods, a single long 

 flagellum. It obviously connects the Mastigophora with the 

 Rhizopoda, and indeed there seems no reason why it should be 

 placed in the present group rather than with the Lobosa. Simi- 

 larly, Dimorpha (5) connects the Flagellata with the Heliozoa : in 

 its flagellate phase (a) it is ovoid and provided with two flagella, 

 but it may send out long stiff radiating pseudopods, while retaining 

 the flagella, or may draw in the latter and assume a purely helizoan 

 phase of existence provided with pseudopods only (6). 



Nuclei of the ordinary character are universally present. In 

 addition there is present in the cytoplasm near the base of the 

 flagellum a much more minute, deeply-staining body, which is termed 

 the UepTiarollast (Fig. 53). This has sometimes been taken for a 

 micronucleus such as is general in the Infusoria, but it is not of 

 nuclear origin, and does not take an active part in any reproductive 

 processes. 



The number of flagella is subject to great variation. There 

 mav be one (Fig. 52, l-$) t two (9, 10\ three (ff), or four (7). 

 Sometimes the flagella show a differentiation in function ; in 



