226 '■^f ANIMAL KINGDOM 



and sometimes even replace a piece containing nuclei, but the replace- 

 ment lacks nuclei. Thus, rotiters are extremely specialized at the cel- 

 lular level, to the extent that further growth and repair are impossible. 

 One oi the challenging unsolved problems of biology concerns the 

 possible differences which may distinguish such nondividing cells from 

 those of other animals. 



86. Senescence 



An individual rotifer lives an active life for only a few days, and yet 

 toward the end of this period it shows several of the characteristic fea- 

 tures of old age. Egg production ceases, the animal becomes sluggish, 

 and portions of the body begin to degenerate. Lansing has found that 

 during these few days the amount of calcium in the body increases, 

 just as it increases with age much more slowly in the bodies of man 

 and other animals. He also found that if the calcium was removed 

 every day by immersing the rotifers in sodium citrate for one minute, 

 the average life span was considerably lengthened. If the aging of rotifers 

 is found to be similar to that of man, they will be used widely in re- 

 search, for mere days rather than years are required for the completion 

 of experiments with rotifers. 



A decline in vigor in successive parthenogenetic generations has 

 been reported in some rotifers. In certain species the appearance of 

 males is not related to external factors but seems to be inherent. After 

 a certain number of female-producing generations, the male-producers 

 appear, resting eggs are produced, and the population disappears for 

 the season. In some, there is a continual decrease in activity and lon- 

 gevity from generation to generation of parthenogenetic females be- 

 fore the sexual phase appears. Little is known of the mechanism by 

 which an aging factor can be transmitted or accumulated through suc- 

 cessive generations. Many rotifer populations do not show this kind 

 of aging, and can be kept as parthenogenetic strains indefinitely. 



87. Resistance to Desiccation 



Perhaps the most interesting aspect of rotifer physiology is the abil- 

 ity of some species, especially those that live in temporary puddles or 

 moss, to resist adverse circumstances. A dry, tarred roof in a hot summer 

 sun, when the tar is bubbling hot, is an unlikely place to find delicate 

 animals, yet if a bit of dried scum is taken from a spot where the last rain 

 puddle dried up, and placed in some fresh water, the dish may be 

 swarming with rotifers within minutes. These are not newly hatched, 

 but are full grown adults. They are visible in the dry scum as rotifer 

 mummies (Fig. 12.4), shrunken bodies with retracted wheel-organs. 

 When water is added they simply swell, stretch out, and begin to move. 

 To see them open out the delicate wheel-organ only minutes after baking 

 in the sun is truly astonishing. 



If well fed rotifers are dried slowly they may survive several years 

 of desiccation. The longest known record is 59 years. In the dry state 

 they can survive extremes of temperature, from well above the l)oiling 



