PROTOZOA 155 



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

 devices become necessary to enable the animals 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. 70) 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 

 almost or wholly destitute of the power of independent motion. 

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

 resting stage, by which they pass uninjured through such 

 unfavorable conditions as drouth, cold, and the like. 



198. Sensation and Behavior. All the Protozoa show more 

 or less sensitiveness to external conditions. They may be 

 caused to contract 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 

 even without any foreign matter whatever. So far as we 

 know, these simple responses do not give evidence of special 

 organs, but merely represent a diffused protoplasmic irritability 

 and power of responding to stimuli (20, 21). 



On the whole, when protozoa are stimulated, their response 

 is an advantageous one. That is it is positive, or toward sub- 

 stances or forces that are of help to it; and negative to stimuli 

 that are hurtful. It is not believed that the protozoan is con- 

 scious of these conditions. It probably means that they have 



