THE MASTIGOPHORA 



121 



(Uroglena, etc.). Again, they may be of equal length but turned in 

 opposite directions (Bodo, etc.). When there are numerous flagella, 

 they may be distributed about the body, regularly or irregularly or 

 aggregated at certain points (Multicilia, Tetramitus, etc.). 



In the Dinoflagellidia, the longitudinal flagellum, a long, fine thread, 

 is invariably directed backward or forward, while the other, the trans- 

 verse, lies around the body in an equatorial groove. This flagellum 

 has a simple undulating motion resembling a row of moving cilia, for 

 which it was at first mistaken. 1 So strictly does the transverse flagel- 

 lum adhere to the usual direction of motion, that even when the 

 groove is absent, as in Prorocentrum, where the flagellum no longer 

 surrounds the body, the motion is retained, 

 the flagellum being directed outward from 

 the end of the body for a short distance 

 and then turned at right angles to form a 

 circle, with the customary undulatory 

 motion, as though still encircling the body 

 (Fig. 66). 



With the exception of the Choano- 

 flagellida, which swim like a spermatozoon 

 with the flagellum behind (James-Clark, 

 '66), the Mastigophora swim with the 

 flagellum in advance. The forward move- 

 ment of most flagellated organisms is, 

 therefore, exceedingly difficult to inter- 

 pret. It is very curious to see the com- d O 

 paratively large body of Peranema, for Fig. 66. Primitive forms of Dino- 

 example, drawn steadily forward by the ^iiidia. [BUTSCHLL] 



' a, Prorocentrum micans Ehr. , 



minute tip of its rather long flagellum. Exuviceiia lima Ehr. 

 No satisfactory mathematical demon- 

 stration of the application of the force necessary to produce this 

 motion has been given. Lankester ('91) compared it to the force 

 produced by a man's arm and hand when swimming upon his 

 side ; Biatschli ('83) offered a simple and apparently reasonable 

 explanation, showing that the resistance, which is directed at right 

 angles to the advancing undulation, can be reduced, through the 

 parallelogram of forces, to a force of rotation and one of translation, 

 but Delage ('96) holds that while this explanation is perfectly con- 

 sistent with the mechanism of certain mechanical contrivances, it is 

 incompatible with the structure of the flagellate body, and that the 

 explanation is much more complicated. Delage's interpretation 

 involves the principles of conic sections, the resisting force being 



1 Hence the name of the group, Cilioflagellata, which was in use until a compara- 

 tively recent date. 



