IS6 ZOOLOGY 



water and thus propel them. In the attached forms contractile 

 strands (myonemes) 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, 

 later becoming attached again. Still other species are encased 

 in shells and are almost or wholly destitute of the power of inde- 

 pendent 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 to 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. Many forms apparently 

 lack special sensory structures and responses seem to be due to 

 diffused protoplasmic irritability alone. In some Infusoria and 

 some Mastigophora a special neuromotor apparatus has been 

 found. 



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 favorable; and negative to stimuli 

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

 scious of these conditions. It probably means that th.ey have 

 inherited the tendencies which through untold generations have 

 resulted in safety. Those with wrong tendencies have been 

 eliminated. In this way, through generations of trial and error 

 and by adjustments on the part of the organisms they have 

 become adapted to the present conditions of life. We have no 



