154 BEHAVIOR OF THE LOWER ORGANISMS 



to turn the organism toward the oral or peristomal side (Fig. loo, 6), 

 Under these circumstances, another principle requires consideration. 

 Normally the peristomal ciha strike backward. When they strike for- 

 ward, they develop much less energy, — less turning power, — than when 

 they strike backward. Therefore, when in the position shown at 6, 

 Fig. IOC, the turning is much less rapid than in other positions, and may 

 easily be prevented by a slight resistance. These relations will be 

 understood by an examination of the diagram (Fig. loo). 



In Paramecium, as we have seen, the same condition of affairs is 

 exemplified to a certain degree, so that the organism turns toward the 



oral side in all positions save from 

 \.M>V.J > - ^ to /, Fig. 63. In the Hypo- 



tricha (Oxytricha and Stylo- 

 I nychia) this condition is most 

 typically exemphfied. A large 

 share of the body cilia are absent 

 or have taken the function of legs, 

 Fig. ioi. — Transverse (or oblique) posi- -^y^hile the peristomal ciha are very 



tion and movement of Oxytricha under the jit i it 



action of the electric current, when the animals powertul. in almOSt all CaSeS 



are in contact with the substratum. The ^J^ggg organisms bcCOme Oriented 

 peristome is directed toward the cathode. , i . i 



to the electric current by turmng 

 toward the aboral (right) side. It is only when the peristomal cilia are 

 squarely facing the cathode (Fig. 100, 6) that the animal may turn toward 

 the oral (left) side. In this position the peristomal ciha beat forward, 

 and all the cilia of the body aid in turning the organism toward the oral 

 side. On reaching a position with anterior end directed to the cathode the 

 peristomal cilia are directed forward, but their beating has become so weak 

 as to be almost without effect. The animal, therefore, retains this position. 

 When specimens of the Hypotricha are in contact with a surface, as 

 is usually the case, the forward beat of the peristomal ciha is often so 

 weak and ineffective in the transverse or oblique position (Fig. 100, 6) 

 that it does not turn the animal against the resistance offered by the 

 attachment of the ventral cilia. Such specimens, therefore, remain in 

 the transverse or obhque position, the anterior end usually shghtly in- 

 clined toward the cathode, as in Fig. loi. In this position they run 

 forward. When the current is reversed, so that the anode hes next the 

 peristome, the powerful peristomal cilia strike backward. The ani- 

 mals, therefore, turn toward the aboral (right) side till they have again 

 become nearly transverse to the current. They then move forward in 

 the direction so indicated. Similar phenomena are at times to be ob- 

 served in other ciliates, not belonging to the Hypotricha. This is true, 

 as we have seen, even for Paramecium. 



