ZOOLOGY. 



192. Movement. The majority of Protozoa move freely 

 in their medium. In Amoeba it is of a gliding character and 

 is effected by putting forth processes into which the protoplasm 

 streams. The process or pseudopodium thus enlarges at the 

 expense of the body of the cell and progress is had in the direc- 

 tion of the growing pseudopodium. The direction of motion 

 is changed by the breaking out of new processes in a new 

 direction. In those Protozoa which have a cell-wall special 

 devices become necessary to enable the animal to move. Most 

 of the free-swimming forms possess cilia or flagella, which 

 act as oars on the water and thus propel them. In Stentor, 

 Spirostomum, Vorticella, etc., there are clearly defined strands 

 of contractile material developed in the ectosarc by which the 

 shape of the animal may be strikingly changed. In the at- 

 tached forms these strands extend from the body proper into 

 the stalk. Vorticella (Fig. 68) by this device may change its 

 position with much suddenness. Attached forms are able to 

 break loose from their moorings and become free-swimming 

 for a time. Still other species are encased in shells and are 

 practically destitute of the power of independent motion. 

 Even the most active types may assume the non-motile or rest- 

 ing stage, by which they pass uninjured through such unfavor- 

 able conditions as drouth, cold, and the like. 



193. Sensation. All the Protozoa show more or less sensi- 

 tiveness to external conditions. They may be caused to con- 

 tract and move by mechanical stimuli such as contact or 

 jarring, by chemically active substances in the water, by light, 

 by changes in temperature, and the like. Vorticella and 

 Spirostomum are exceedingly sensitive to contacts; Amoeba 

 avoids the light; many forms seem to find their food as the 

 result of the chemical differences in the water and may be 

 seen to swarm about suitable objects ; the contractile vacuoles 

 of many forms contract more rapidly in warm than in cold 

 water ; Paramecia tend to collect in groups at the edge of the 

 cover glass, around air-bubbles, about green filaments, or 



