28 HOMOIOTHERMISM \ 



water over 25 to 30 deg. C, and apparently never lived 

 at temperatures of more than 40 deg. C. Tadpoles, how- 

 ever, were found living in water very near this limit. 



It is a very interesting ecological point that nearly 

 all groups of animals that are represented in freshwater 

 contain a few species known to inhabit hot waters. The 

 forms that adjust their tolerance to high temperatures 

 are extremely varied. 



Aquatic animals may have great difficulty living in 

 warm water because of the small amount of dissolved 

 oxygen. Many of the species which inhabit hot springs 

 obtain their oxygen from the air. It is perhaps signifi- 

 cant that the animals living in such situations where the 

 essential elements, salts, oxygen, ^^H, etc., may be very 

 unlike the ordinary freshwater conditions, show a greater 

 resemblance to freshwater forms than do the marine 

 forms which have always lived in the sea, where they 

 are never obliged to resist great fluctuations in the tem- 

 perature of their environment. Apparently animals 

 which now live in waters which have high temperatures 

 have been derived rather recently from typical fresh- 

 water animals and not from their more marine ancestors. 



Body Temperature of Poikilotherms. — While it ap- 

 pears from the observations of many of the older ob- 

 servers that the body temperature of poikilotherms may 

 differ as much as several degrees from that of the en- 

 vironment, later workers have found that such tempera- 

 tures may resemble each other very closely, even to 

 hundredths of a degree. Eogers (1927) found that the 

 earthworm, salamander, clam, and goldfish in rapidly cir- 

 culating water, had body temperatures of almost exactly 

 the same temperature as the surrounding water. In the 

 experiment summarized in Fig. 1 the body temperature 

 of a turtle soon came to the same temperature as the 



