BEHAVIOR OF THE LOWER ORGANISMS 



FIG. 77. Reaction of Bursaria, ventral view. 



times of most of the organisms 

 It is much more usual in cer- 



turning toward the right side (Fig. 77). Spirostomum ambiguum and 

 Spirostomum tenue swim backward and turn toward the aboral side. 

 Opalina ranarum turns toward the more convex (right) side, Nycto- 



therus toward the aboral side 

 (Fig. 78). Many of these or- 

 ganisms show an additional 

 reaction to strong stimuli, con- 

 sisting in a marked contraction 

 of the body. This is particu- 

 larly noticeable in Spirostomum 

 and Stentor. 



Many of the Ciliata do not 

 as a rule swim freely through 

 the water, but creep along sur- 

 faces, keeping one side against 

 the surface. This is true at 



1-5, successive positions occupied. 



mentioned in the foregoing paragraph, 

 tain other ciliates, belonging to the group of Hypotricha (Fig. 31, /; 

 Fig. 81). In these animals the cilia of one side of the body are spe- 

 cially modified for creeping, while the opposite side 

 bears either few and weak cilia or none at all. The 

 Hypotricha are usually found running about on the 

 bottom, or on the surface of objects in the water. 

 In addition to their creeping movements, they pro- 

 duce by means of strong peristomal cilia a vortex 

 leading back to the mouth. These animals of course 

 do not revolve on the long axis as they progress, and 

 the corresponding feature is likewise lacking in the 

 reactions to stimuli. On coming in contact with an 

 obstacle, or when otherwise stimulated, they stop or v FlG - ?? ~~ Nvct - 



* therus. The arrow to 



move backward a distance, then turn toward a cer- the right shows the 

 tain structurally marked side, keeping in contact with direction of turning in 



* . . response to stimula- 



the substratum and not revolving on the long axis, tion, while the three 

 This renders it much easier to observe the precise ** arrows indicate 



.. . * the direction of beat of 



method of reacting than m Paramecium, where the the cilia. After Dale 

 rapid revolution on the long axis is very confusing, ('9 01 )- 

 As examples of the creeping infusoria, the following may be mentioned : 

 Stylonychia (Fig. 31, /), Oxytricha, and other Hypotricha react to 

 most stimuli by moving backward and turning to the right (Fig. 79). 

 These organisms are particularly favorable for the study of the reaction 

 method. The body is flat, and the right and left sides are very easily 



