DERIVED ORGANIZATION— TAXONOMIC STRUCTURES 141 



laria, to the typical motile organ of the flagellate type? Or is it 

 more probable that a motile organ originating from a definite kinetic 

 center (basal body or blepharoplast) has become progressively indefi- 

 nite with loss of the kinetic elements through the same series of 

 forms, but in the opposite direction, and ending in types like Amoeba 

 proteus? To my mind, the pseudopodia of Amoeba proteus and its 

 immediate relations, have no place at all in such a series; they are 

 merely expressions of the physical conditions of the protoplasm and 

 of the forces operating within, and they may appear in any cell 

 having an appropriate physical make up. Thus we find them in 

 certain types of cell (leukocytes and phagocytes) widely distributed 

 throughout the animal kingdom, and we find them here and there, 

 in every group of the Protozoa. 



An illuminating illustration in support of this conclusion is 

 afforded by the transitory flagellated stages of one group of ameboid 

 organisms, the Bistadiidae (see p. 108). Here, in Dimastigamoeba 

 gruberi, for example, the organism loses its pseudopodia under cer- 

 tain conditions, and develops flagella, not by metamorphosis of the 

 pseudopodia, but from blepharoplasts which, as centrioles, emerge 

 from the nucleus (Fig. 59, p. 108). 



Although only a matter of academic interest, I believe that the 

 flagellum 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. 120). 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. 411). 



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 

 many types of Metazoa. They are sometimes combined with 

 pseudopodia (Dimorpha mutans, Fig. 13, p. 34, Mastigamoeba inver- 

 tens, Ciliophrys infusionum, etc.), sometimes with cilia (Myriaphrys 

 paradoxa, Fig. 197, p. 478). 



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 periplastic substance of the cor- 



