ORGANS OF LOCOMOTION OF PROTOZOA 35 



subjects of contention between bacteriologists and protozoologists. 

 Little satisfaction, however, comes from such wrangling, and there is 

 little practical value in connection with these hypothetical boundary 

 lines beyond setting the limits to text-book or monograph. 



Pseudopodia, and Classification of the Sarcodina. In many 

 respects pseudopodia are the simplest forms of motile organs. They 

 are merely prolongations or outflowings of the cell protoplasm, the 

 external expressions of internal physical forces which biologists have 

 tried in vain to analyze. In the inner protoplasm of nearly all kinds of 

 protozoa, the almost fluid cell contents with granules of various kinds, 

 food more or less digested, and with waste products, are in a constant 

 movement or cyclosis. In the more highly differentiated forms of 

 protozoa, this flow is quite confined to the inner protoplasm, the firm 

 cell membrane preventing an outward manifestation of the forces 

 which cause the flow. In the shell-less Sarcodina, however, there is no 

 firm outer covering, and the peripheral protoplasm gives way at the 

 points of least resistance and an outward flow of protoplasmic stuff 

 is the result, this flow ceasing with the exhaustion of the particular 

 force which caused it, while a new point of rupture gives rise to a new 

 pseudopodium. Thus the motile organs of these low types are incon- 

 stant, endlessly changing centres of protoplasmic energy, which have 

 defied the physicist, the chemist, and the biologist. Xot all pseudopodia 

 are of this simple type, however, and some of them have a permanent 

 form with supporting skeletal elements. The former, transitory kind, 

 are characteristic of the ordinary rhizopods such as ameba, arcella, 

 difflugia, etc., which are familiar to the novice as "the lowest forms 

 of animal life," and they appear and disappear again with an ever- 

 fascinating, inexplicable regularity. These are the so-called lobose, 

 "lobopodia," or finger-form pseudopodia. 



The second, more permanent kind of pseudopodia, are sometimes 

 called axiopodia, because of the presence of a stiff axial filament, com- 

 posed of condensed protoplasm similar to acanthin or chitin, which 

 runs through the axis of the pseudopodium. These pseudopodia, 

 characteristic of the class Actinopoda, stand out, ray-like, from all 

 sides of the usually spherical animal, and give a peculiar radiating 

 appearance which led the early students of the group to call them the 

 sun-animals, a name which Haeckel, with characteristic felicity, turned 

 into Heliozoa. In these the protoplasmic flow leads to no change in 

 configuration of the motile organ, but courses outward on one side of 

 the pseudopodium and backward on another. 



The central axis belonging, as shown above, to the category of kino- 

 plasmic substances, has a certain amount of elasticity, and may bend 

 and straighten again with considerable force, and thus the pseudo- 

 podium becomes a more or less vigorous organ of locomotion, an 

 acanthocystis rolling over and over with a slow vibration of the elastic 



