74 PROTOZOA 



is the same as that seen in the "trial-and-error" mode of behavior of 

 higher animals, and so this term is often appHed to the activity of this and 

 similar Protozoa. 



Whenever a Paramecium is responding to one stimulus it often will 

 not be affected by another stimulating agent unless the second one is 

 very strong. It has been found, however, that the response to gravity 

 is always set aside whenever the animal receives any other stimulus. 



The responses which paramecia give to stimuli are not always the 

 same, the difference being due to the different 'physiological states of the 

 animals. A Paramecium which is fully fed tends to come to the surface 

 and remain quiet in contact with some object, though the sources of its 

 food supply may be, in a laboratory culture, at the bottom of the jar. 

 When it becomes hungry, however, it reverses its responses, swims to the 

 bottom, secures its food, and then once more seeks the surface. Thus one 

 physiological state gradually becomes changed into another, and a definite 

 rhythm is established in the animal's movements. It has been found that 

 a response becomes more pronounced after it has taken place a number of 

 times. This indicates a change in physiological state and shows an effect 

 of one response upon succeeding ones which has been termed summation of 

 stimuli. 



When violently stimulated either by a chemical or by contact, a 

 Paramecium frequently responds by throwing out the contents of the 

 trichocysts, which harden and form a barrier of fine threads. When 

 the trichocysts are. emptied, they are refilled by material which originates 

 near the nucleus, probably from it, and passes through the endoplasm to 

 the proper points in the ectoplasm. 



104. Reproduction. — Paramecia reproduce only by fission, the animal 

 being divided transversely into two. During this process both the 

 macronucleus and the micronucleus divide, the old gullet divides into 

 two, and two new contractile vacuoles arise by division of the old ones. 

 The micronucleus divides mitotically, and perhaps the macronucleus does 

 also. The entire process occupies from half an hour to two hours. Sub- 

 sequent growth is rapid, and division occurs again after a number of hours. 

 Paramecia multiply with great rapidity. It has been estimated that from 

 one ancestor could be produced in one month, if all survived, a total of 

 265,000,000 individuals. 



105. Conjugation. — At intervals occurs a phenomenon known as con- 

 jugation (Fig. 27). When this occurs, two paramecia come together, 

 attached by the surfaces on which the oral grooves are located. The 

 pellicle breaks down at the point of contact, as does also the ectoplasm, 

 and an endoplasmic bridge is formed between the two animals. During 

 this time the micronucleus of each conjugant moves from the concavity 

 in the macronucleus, where it has been lodged, grows larger, forms a 

 spindle, and divides. A second division follows immediately. Of the 



