REACTIONS TO HEAT AND COLD. 23 



This continues until the heat becomes destructive the animals cease 

 circling, become quiet, and finally disintegrate. The reaction of those 

 individuals which are resting or creeping on the bottom is thus not of 

 a character to save them from destruction. 



Specimens which are by chance moving along the bottom from a 

 cool region toward the warm region do not escape ; they merely stop 

 and begin to circle backward to the right when they reach the heated 

 spot, and continue this till they die. 



Thus the reaction of Bursaria to heat, while of the same general 

 character as that of other infusoria, must be accounted very imperfect, 

 since it hardly results in orientation at all, and does not preserve the 

 animals from destruction. 



Paramecium caudatum:* In the second of my studies (Jennings, 

 1899, pp. 334-336) I gave a brief account of the way in which, ac- 

 cording to my observations, Paramecium reacts to heat and cold. 

 From my more recent studies I can confirm this account. But as 

 Mendelssohn has recently come to different conclusions for the tempera- 

 ture reaction of this animal, and as he misunderstands certain points 

 in my brief description, it seems desirable that I should supplement the 

 account previously given in order to make it clear. 



Paramecium reacts to heat and cold in essentially the same manner 

 as is described above in detail for Oxytricha. When the higher or 

 the lower temperature advances from one side the animals swim 

 backward, turn toward the aboral side, and swim forward again. 

 They continue this until the movement brings them into a region of 

 more moderate temperature. Paramecium reacts more readily than 

 Oxytricha, the reactions are repeated at shorter intervals, and the 

 movements are more rapid, so that a common orientation of many 

 individuals swimming away from the region of higher or lower tem- 

 perature is more quickly produced and is more striking to the eye. It 

 results farther from this more rapid movement, as well as from certain 

 other factors, that the method of reaction in Paramecium is much less 

 easily observed than in any of the other infusoria described. Indeed, 

 Paramecium is one of the most unfavorable forms obtainable for a 

 study of reaction methods, and it is, I believe, due largely to the fact 

 that this animal is usually employed for such study that progress has 



*The common Paramecium, which appears everywhere in immense numbers 

 in decaying vegetation, receives from different authors sometimes the name 

 Paramecium aurelia, used by Mendelssohn ; sometimes the name given above. 

 I use the name caudatum because it appears to me to be the correct one, but 

 there is no reason for considering the animals thus differently denominated to 

 be really different. 



