136 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS 



General Notes on the Functions of Protozoa 



Movement. — The simplest form of movement is that 

 termed amoeboid, as illustrated by an Amoeba. In ordinary 

 conditions it is continually changing its shape, putting 

 forth blunt lobes and drawing others in. Surface tension 

 phenomena occur on the outermost zone of the cytoplasm, 

 and also beneath that — often along the dorsal surface in the 

 direction of motion, then over the front end, then along 

 the ventral surface next the substratum, then again 

 at the posterior end dorsally. Thus there is a complex 

 " caterpillar- wheel "-like stream.ing movement of the 

 granules. No final explanation of the whole process in 

 physico-chemical terms has yet been given. A more 

 defined contraction, like that of a muscle cell, is illustrated 

 in the contractile filam.ent of the stalk of Vorticella and 

 similar Infusorians ; and not less definite are the move- 

 ments of cilia and flagella, by means of which most In- 

 fusorians travel swiftly through the water. Cilia in 

 movement are bent and straightened alternately ; while 

 flagella, which are usually single mobile threads, exhibit 

 lashing movements to and fro, or, more often, are held 

 stretched out in front, and by a curious rotatory movement 

 draw the cell along. They are then more aptly termed 

 tractella. It seems probable that cilia and flagella consist 

 of an elastic core surrounded by a sheath, which may be 

 uniformly contractile, or may have one contractile band, or 

 two opposite contractile bands, and so on. 



Considered generally, the movements are of two kinds : either (i) re- 

 flex, i.e. responses to external stimulus, as when the Protozoon moves 

 towards a nutritive substance ; or (2) automatic, i.e. such movements as 

 appear to originate from within, without our being able to point to the 

 immediate stimulus, e.g. the rhythmical pulsations of contractile 

 vacuoles. Actively moving Protozoa usually show the following motor 

 reaction to stimulus : — they move backward, turn over on one side 

 structurally defined, and then move forward again. 



Sensitiveness. — The Amoeba is sensitive to external in- 

 fluences. It shrinks from strong light and obnoxious 

 materials ; it moves towards nutritive substances. This 

 sensitiveness is, so far as we know, diffuse — a property of 



