134 BIOLOGY OF THE PROTOZOA 



pseudopodia, but from blepharoplasts which, as centrioles, emerge 

 from the nucleus (Fig. 42, p. 86). 



For these reasons I beheve that the flagelhnu type of motile organs 

 is the most primitive type we know while axopodia and myxopodia, 

 the former with kinetic elements of weakened function, the latter 

 with denser axial protoplasm which Doflein also interprets as 

 equivalent to axial filaments, represent stages in the deterioration 

 of the kinetic function coincident with the absence of definite kinetic 

 centers (see also p. 141). For these reasons also, together with 

 others which will be given later, we hold with Doflein (1916), 

 Klebs and many others, that the group of flagellates furnishes more 

 evidence of original ancestry than do the rhizopods (see p. 251). 



1. Flagella.— Flagella are widely distributed throughout the 

 animal and plant kingdoms, forming the motile elements of animal 

 spermatozoa and of plant zoospores, or current-producing organs of 

 the collared cells of sponges. They are sometimes combined with 

 pseudopodia {Dlmorpha mutans, Fig. 12, p. 34, Mastigamoeba 

 invertens, Fig. 137, p. 287, Ciliophrys infusionum, etc.), sometimes 

 with cilia {Myriaphrys paradoxa, Fig. 160, p. 369). 



Flagella are usually excessively fine and delicate fibers extremely 

 difficult to see and to study in the living organism. In the great 

 majority of cases the finer structure has not been made out, but in 

 a few favorable types some progress has been made. In these cases 

 it is known that the flagellum is made up of two definite elements, 

 an axial, highly vibratile filament, which is formed as an outgrowth 

 from the basal body or blepharoplast, and an enveloping elastic 

 sheath which is formed from the protoplasmic substance of the 

 cortex. 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- 

 aiiema trichophora or certain species of Astasia, 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 M'hich are band form (some species of Dinoflagellida). Such 

 band-form flagella suggest the possibility that vibratile membranes, 

 which are not uncommon in parasitic types of flagellates, may; mor- 

 phologically, be regarded as flagellum sheaths which remain attached 



