142 BIOLOGY OF THE PROTOZOA 



tex. In some cases the sheath is circular in cross-section (see 

 Plenge), in others ellipsoidal, while the contractile thread which is 

 usually attached firmly to the sheath may run in a straight line the 

 entire length of the sheath, or may follow a spiral course. In the 

 majority of flagellates the sheath undulates and vibrates in unison 

 with the contractile axial thread, but in a few types, such as Per- 

 anema trichophora or certain species of Bodo, the sheath remains 

 passive while the axial thread extends freely beyond the limits of 

 the sheath, where its activity in the surrounding medium results in 

 a steady progressive movement of the cell. Under the influence of 

 somewhat violent stimuli, however, the sheath itself may undergo 

 fibrations in such forms. 



Owing to the nature of flagella and to their delicacy of structure, 

 there are not many possibilities of variation in type. In addition 

 to those which are circular or ellipsoidal in cross-section, there are 

 some which are band form. Such band-form flagella suggest the 

 possibility that vibratile membranes, which are not uncommon in 

 parasitic types of flagellates, may, morphologically, be regarded as 

 flagellum sheaths which remain attached throughout their length 

 to the cortex while the axial thread forms the contractile margin 

 (Fig. 169, p. 360). Such vibratile membranes are characteristic of 

 the genera Trypanosoma, Cryptobia, Trichomonas, Trichomastix, etc., 

 all of which are parasites in the blood or digestive tract of different 

 animals. 



There are, however, abundant variations in size, number and 

 position of flagella in the cell. When there is but one it usually 

 emerges from a pit or funnel-shaped opening at the anterior end of 

 the cell (flagellum fissure). When two are present they may be 

 equal in size and length (e. g., Spongomonas splendida, Fig. 49, p. 

 95), or one may be considerably thicker and longer than the other 

 (heteromastigote types). Both may be directed forward as in 

 Amphimonadidae or one may be directed forward, the other back- 

 ward, as in Bodo, Anisonema, etc. In such cases the posteriorly 

 directed flagellum (trailing flagellum or Schleppgeissel) appears to 

 act as a runner upon which the cell body glides, and has little to do 

 with the actual locomotion of the animal (Fig. 76). 



Delage and Herouard have attempted to explain the dynamics of 

 flagellum action whereby the comparatively heavy body is moved 

 forward by reason of the vibrations of the exceeding^ delicate 

 thread. In the usual type the extremity of the flagellum describes 

 a rather wide circle so that it is in a certain focus of the microscope 

 for only an instant of time. With this circular movement, which 

 varies in different species, constant undulations pass from the base 

 to the tip. A forward pull results from the combination of such 

 movements and the cell either glides smoothly after its active pro- 

 peller or rotates more or less rapidly on its long axis while freely 



