102 



BEHAVIOR OF THE LOWER ORGANISMS 



All together, it is clear that there are differences in behavior due to 

 differences in the internal or physiological condition of the animal, — 

 differences shown even in a single individual at different times. Some 

 of the different physiological conditions may be characterized as fatigue, 

 as acclimatization, as hunger, or the like. In other cases they cannot 

 be definitely characterized. We clearly have slight beginnings of the 

 modification of behavior through the previous experiences of the organism. 

 The analysis of this matter will be carried farther for the behavior of 

 unicellular organisms in the account of Stentor (Chapter X). 



3. BEHAVIOR IN FISSION AND CONJUGATION 



At intervals certain extraordinary episodes connected with the pro- 

 cesses of reproduction interrupt the usual life of Paramecium. The 

 behavior at such times seems not to differ in any notable manner from 

 the usual behavior. We shall therefore describe it only briefly. 



Fission. — At times the animal begins to divide into two by a trans- 

 verse constriction at about the middle. During the early stages of the 

 process the two halves act in unison. The currents 

 of water are driven by the cilia in the same direction 

 over both, and the two halves react to any stimulus 

 as a single animal. If subjected to induction shocks 

 the half at the anode responds by contraction of the 

 ectosarc and discharge of trichocysts, while the 

 cathode half does not. As the constriction sepa- 

 rating the two halves becomes very deep, so that they 

 are connected only by a slender strand, they begin 

 to behave more independently. The anterior half 

 at times changes its direction of movement, while the 

 posterior half tries to continue straight forward. The 

 connecting strand is strained and bent or twisted. 

 Soon it breaks, and the two individuals are separated. 



Conjugation. — In conjugation two individuals become united by 

 their oral surfaces (Fig. 69), and a complicated process of interchange 

 of nuclei occurs. The union of two specimens seems brought about 

 chiefly by the usual movements and reactions of the animals, taken in 

 connection with a physical change of the body substance in the region 

 of the oral groove. Here the surface becomes viscid, so that if another 

 Paramecium comes in contact with this region, the two stick together. 

 Often two individuals may be seen at rest close together on the surface 

 of a bit of bacterial zooglcea. One drags its posterior end across the 

 oral groove of the other, whereupon the two stick together (Fig. 70, a). 



Fig. 60. — A pair 

 of conjugating Para- 

 mecia. 



