KESISTANCE TO DEOtTGHT. 175 



during prolonged drought, to plants, stones, &c., or bury them- 

 selves in the soil and close the mouth of their shell with a 

 calcareous deposit known as the diaphragm ; thus they await 

 the next rainy season to recommence an active life. Here it is 

 easy to prove that the animals are not truly desiccated ; for if 

 we break into the shell of a snail thus found in its summer 

 sleep, we see at once that the creature has pi-eserved a very 

 considerable amount of moisture, which the hygrosoopically dry 

 air has not been able to evaporate from the animal, jwoteoted as 

 it is by its shell and diaphragm. 



It is to this property possessed by living animals of retaining 

 a certain amount, however small, of moisture for a long time 

 in their tissues, and consequently of escaping total desiccation, 

 that the power is evidently due which enables the eggs and 

 germs of the above-mentioned animals to continue all the year 

 round ^^ in an apparently dry condition withont being deprived 

 of their vitality. It is certainly very striking that encysted 

 Infusoria, and the ova or reproductive bodies of Crustaceans, 

 Tardigrades, "Worms, Sponges, &c., are capable of withstanding 

 the long-continued desiccating effects of the air ; but if at the same 

 time we remember that it is extremely difficult to desiccate albu- 

 minous matter completely, even when dead, the fact loses some- 

 thing of its astonishing character. Living plants, too, often 

 retain the last remnant of their moisture with much obstinacy. 



Of the truth of these facts there certainly cannot be the 

 smallest doubt. I have had for now six years a chest full of 

 dried mud with the eggs of Apus and Gypris, which were sent 

 to me in the spring of 1872 by Ehlers from Erlangen. Tip to 

 the present time every attempt to hatch out some of the 

 Apus larvae by softening a part of the mud has succeeded 

 equally well in summer and in winter; the rapidity of their 

 development is different, but this is due, as we have seen in a 

 former chapter, to the degree of temperature at the time. 



Now, remarkable as is this long resistance of eggs to 

 drought, we are acquainted with a much more wonderful, 

 imd, in fact, hitherto inexplicable, fact connected with it. The 

 eggs of various Crustaceans — as, for instance, of species of 

 Apus — never develope '^ if they have not first lain some time in 



