THE MASTIGOPHORA 159 



supposed, since the phenomenon of parallelism is as strikingly dis- 

 played in this respect as in the manner of formation of colonies. 



The heteromastigote condition merits particular notice since it 

 characterises an entire sub -class (Dinoflagellata), where the one 

 flagellum is transverse, usually lying in an annular depression, while 

 the other is longitudinal and is also partially protected by a groove, 

 but extends backwards freely (Fig. 10). This is a special mani- 

 festation of the heteromastigote condition, but equally interesting 

 examples occur in many families of Lissoflagellata, where the 

 anterior flagellum is normally directed forwards (tractellum) and 

 the posterior flagellum which arises from the body of the cell close 

 to the former is trailed behind. The posterior flagellum in these 

 cases exerts a directive and modifying influence upon the move- 

 ments of the Infusorian, serving also as an anchor and sometimes as 

 a spring promoting a rapid jerking movement of leaps and bounds 

 like the tail of a Podurid. 



The posterior flagellum of heteromastigote Mastigophora may 

 be aptly described as a gubernaculum (Fig. 7(10)) and referred to by 

 that term. 



The flagellum is usually so extremely attenuated that it is 

 very difficult to discover any structure in it, but as its base 

 may often be traced from the surface through the ectoplasm to the 

 endoplasm, it seems probable that it 

 consists of an axial filament derived 

 from the endoplasm and a delicate 

 cortical sheath derived from the ecto- 



plasm. It is interesting to note that $*w&& IA ax. 



in the Rhizomastigoda there is an 

 endoplastic axial filament in the pseudo- 

 podia (Fig. 1). It is impossible to 

 draw any morphological line of dis- 

 tinction between a flagellum and a . , 



cilium, and in the Lophomonadina, for 

 example, the vibratile processes have Fia L 



been interpreted as flagella by those 



who regard this group as belonging a*J*SS ta 



A , X/r i -, .,. \ ena ) expending into the ectoplasm 



to the Mastigophora and as cilia by (cOofthepseudopodia(ps);/, flagei- 

 those who regard it as a family of IS ^' esa^A^-' GoifimMt?)" 

 Infusoria. Since the discovery that 



the equatorial groove of the Dinoflagellata (p. 182) is not ciliated, it 

 is usually regarded as a character of the class that true cilia do not 

 occur ; and if the vibratile processes of the Polymastigina are true 

 flagella, the only exception to this is to be found in the aberrant 

 genera Pteridomonas, Maupasia, and Monomastix (pp. 164 and 170). 

 As a rule, there seems to be no connection between the base of 

 the flagellum and the nucleus, but such a connection can be traced 



