112 



BEHAVIOR OF THE LOWER ORGANISMS 



Paramecium, the method running through the behavior is that of the 

 selection of certain conditions through the production of varied move- 

 ments. When stimulated the animal "tries" many different directions 

 till one is found in which stimulation 

 ceases. This reaction is known to be pro- 

 duced in Chilomonas by heat, by the 

 drying up of the water containing the 

 animals, by mechanical stimulation, by 

 various chemicals, by passage from water 

 containing certain chemicals (acid) to water 

 containing none, and by the electric cur- 

 rent. We shall take up certain details of 

 the reactions of Chilomonas in the sec- 

 tions which deal with the 

 different classes of stimuK. 

 Euglena. — Euglena 

 viridis (Fig. 74), Hke 

 Chilomonas, swims in a 

 spiral. The larger lip 

 "^ (Fig. 74, x) is always 

 toward the outer side of 

 the spiral (Fig. 94). 

 When stimulated by com- 

 ing in contact with a weak 

 chemical, by a mechanical 

 shock, or by a change in 

 the intensity of Hght, Eu- 

 glena responds by an 

 avoiding reaction similar 

 to that of Paramecium 

 and Chilomonas. The 

 Fig. 74. — £«- forward motion becomes 



glena vtrtdts, after 



Kent. I,, v., reservoir slowcr, ceascs, or (more 

 of the contractile vacu- rarely) is transformed 



ole; e, eye spot; g, . ^ , , , 



guUet; nu, nucleus; x, mto a backward motion 



larger or upper lip. Then the 



nu. 



Fig. 73. — Spiral path of 

 Chilomonas. a, b, c, d, suc- 

 cessive positions occupied. 



organism 



swerves more strongly than usual toward the larger lip. Thus the 

 spiral becomes wider and the organism becomes pointed successively 

 in many directions (see Fig. 91). In one of these directions it finally 

 swims forward, repeating the reaction if again stimulated. We shall 

 have occasion to describe in detail the reactions of Euglena to light 

 (Chapter VIII). 



