140 BIOLOGY OF THE PROTOZOA 



fication which Dujardin first em])loyed to create the three great 

 groups les flagelles, les rhizopodes, and les cilies. Each type is sub- 

 ject to many variations, due to inherent differences in the motile 

 organoids themselves, or to fusion in various ways leading to struc- 

 tures of considerable complexity. 



It is extremely difficult to decide whether flagella or pseudopodia 

 are the more primitive in type. From most general text-books on 

 Zoology we learn that the matter admits of no question, and are 

 taught that the pseudopodium is the most primitive form of motile 

 organ in the animal kingdom. This certainly has been the most 

 widely accepted view. Many a generalization referring to Protozoa, 

 however, which has found its way into general works on Biology, 

 appears to have been drawn from the conditions in some one organ- 

 ism which is conspicuous by reason of its abundance and ease of 

 study. It would sometimes appear, indeed, that the common 

 species of Paramecium and Amoeba proteus, to many general writers 

 constitute the Protozoa. This seems to be the case with the problem 

 of pseudopodia and flagella, the argument being that a pseudopo- 

 dium of Amoeba proteus is certainly a less complex motile organ 

 than the flagellum of Euglena viridis, and therefore more primitive. 

 Had the comparison been made between the pseudopodia of Actino- 

 phri/s sol or Acanthocystis aculeata and a typical flagellum, the con- 

 clusion would not have been so obvious. There is a good deal of 

 evidence against the generalization as it is usually expressed. In 

 the first place, a pseudopodium of Amoeba proteus cannot be inter- 

 preted as a motile organ. It is not a definite structure in the cell, 

 nor does it cause the body of Amoeba proteus to move. On the con- 

 trary, it exists because of the movement of the body protoplasm 

 and the pseudopodium is merely the visible, physical expression of 

 this movement which, in turn, is due to the transformation of energy 

 in destructive metabolism. This energy finds its vent in that por- 

 tion of the ectoplasm which, for the time being offers the least resist- 

 ance; the ectoplasm gives way at this point, the endoplasm gushes 

 through and a pseudopodium results (see Chapter XII, p. 435). 

 Such pseudopodia are not the source of movements of the cell, 

 they are results, not causes, of movement. The pseudopodia of 

 some Heliozoa, on the other hand, are motile organs, and the axial 

 filaments which they contain are regarded as equivalent in struc- 

 ture and in mode of origin to the kinetic elements of flagella. The 

 pseudopodia of Foraminifera are intermediate between those of 

 Heliozoa and those of testate rhizopods. The problem, then, comes 

 down to a theoretical question of probabilities. Is it more probable 

 that pseudopodia of the type found in Amoeba proteus become pro- 

 gressively differentiated into motile organs through stages like the 

 finger-formed pseudopodia of the testate rhizopods, the reticulate 

 pseudopodia of Foraminifera and axopodia of Heliozoa and Radio- 



