86 HOMOIOTHERMISM 



inhabit certain levels where the temperature range fa- 

 vors their metabolic activities. Some poikilothermic 

 animals are very quick to perceive variations away from 

 their optimum and quickly respond in such a way as to 

 remain in favorable temperatures. This is true of her- 

 ring (Powers, 1923), migrating salmon (Ward, 1921), 

 various plankton animals (Pearse, 1926), animals in 

 deserts (Buxton, 1923), and bats which live in caves 

 (Halm, 1908). 



The behavior of many poikilotherms changes mark- 

 edly with variations in temperature. A frog, for ex- 

 ample, at temperatures above 10 deg. C. is usually 

 strongly positive in its responses to light, but if the tem- 

 perature falls below that, the frog becomes negative and 

 if given an opportunity will burrow or crawl under some 

 object, and at temperatures near deg. C. becomes more 

 or less torpid (Torelle, 1903). Sudden changes in tem- 

 perature will cause a reversal of the usual responses of 

 various animals to light (Mast, 1911). Such responses 

 are usually adaptive. Among organisms which are nega- 

 tive to light at low temperatures Washburn (1926) men- 

 tions swarm spores, certain protozoa, an amphipod, and 

 certain aquatic insects ; among those which are positive 

 at lower temperatures, certain copepods and annelid 

 larvae. When planarians are subjected to increasing 

 temperatures, between 23 and 26 deg. C. they become 

 quite active and are positive to light ; between 26 and 38 

 deg. C. they continue to crawl actively but become nega- 

 tive to light ; and at 38 to 39 deg. C. the crawling becomes 

 extremely active and is accompanied by violent twisting 

 and turnings. Perhaps the sensations experienced are 

 increasingly unpleasant. 



A homoiotherm which is insulated with feathers or 

 hair to prevent the loss of heat and has mechanisms to 



