120 THE PROTOZOA 



of one thickness throughout. In either case the base is inserted in 

 the vicinity of a pharyngeal depression and usually at the end of the 

 body. There is good reason to believe with Klebs, Frenzel, Bloch- 

 mann, and others, that like the axial filaments of the pseudopodia in 

 Heliozoa and Radiolaria, the flagellum originates at or near the 

 nuclear membrane, and does not consist exclusively of the outer or 

 peripheral plasm. Dallinger ('78) asserts that the newly forming 

 flagella are smooth and uniform, arising in or near the nucleus. 

 Fischer ('94), however, claims that many of them are provided with 

 branches like the cilia of a typhoid germ. He finds others which are 

 not vibratile throughout their entire length, but are rigid and uniform 

 for a certain distance and then taper to the extremity. Such flagella 

 resemble a whip stock and its lash, the relative proportion of stock 

 and lash varying in different flagella, the stock sometimes running 

 nearly to the end, and again only a short distance from the body. 

 Other forms, especially in the Dinoflagellidia, have spirally rolled 

 flagella of various kinds, while some have flattened or band forms 

 (JPeridinimn tabulatum and P. divergent}. 



A difference of opinion exists as to the ability of the organism to 

 absorb or retract its flagellum into the body-protoplasm. Most 

 observers agree with the early observation of Dujardin that there is a 

 close relation between pseudopodia and flagella, numerous observa- 

 tions having been recorded of cases where, under certain conditions, 

 the pseudopodia change into swinging flagella, and flagella into pseu- 

 dopodia. There is no doubt that flagella can be absorbed after 

 changing to pseudopodia. Whether the fully formed flagella can be 

 changed over into plastic material and then withdrawn, is still a sub- 

 ject of dispute; Fischer ('94) holds that they are invariably discarded 

 upon irritation, and Schiitt ('95) shows that the longitudinal flagellum 

 in the Dinoflagellidia is thrown off upon irritation, while the horizontal 

 flagellum is flattened into a band form. A general rule, therefore, 

 cannot be formulated m regard to the disposal of flagella. In some 

 cases they are absorbed ; in others, thrown off. 



The action of the flagella varies with the type of structure. In the 

 simple, straight, or tapering forms the tip moves in a circle while 

 waves pass from the base to the extremity. In the whip-like flagella 

 the basal portion moves back and forth or in a circle, while the distal 

 region vibrates or undulates like the snapper of a whip. The band- 

 formed flagella move by simple undulations. 



The position of the flagella is extremely variable. When there is 

 but one, it is found at the anterior end of the cell, that is, the end 

 which is directed forward when in motion. When there are two fla- 

 gella, they may both be directed forward (Chilomonas> Cryptomonas, 

 etc.), and may be of equal (Cryptomonas, etc.) or unequal length 



