No. 502] THE DESICCATION OF ROTIFERS 669 



ably develop however from the "winter eggs"; thick 

 shelled fertilized eggs, which in some cases are known to 

 withstand prolonged desiccation. 



During this summer some winter eggs of Asplanchna 

 brightwellii and Hydaiina scuta, which had been laid in 

 June, were kept in water taken from the culture 6 jars 

 until August 3. Then they were taken out with a little 

 sediment and allowed to dry. On August 5, the sediment 

 was apparently thoroughly dried. On August 10 spring 

 water was added and at the end of twenty-four hours 

 several small Asplanchna were swimming about in the 

 water. Later young Hydatina were found in the water. 

 The eggs seem to vary much in the length of time re- 

 quired for them to hatch, some not hatching for three 

 or four days after being placed in spring water while 

 others hatch within twenty-four hours. This may be due 

 to differences in the rate of rapidity in which water pene- 

 trates the egg membranes. In sections of the winter eggs 

 of Hydatina scuta it is very noticeable that the thickness 

 of the outer egg membrane varies greatly in different 

 eggs. 



On August 4 ten to fifteen cubic centimeters of mud 

 and sediment were collected in a finger-bowl from the 

 pond in which Asplanchna bnyhtn-cllii, Branchionus 

 urceolaris, and Pedalion mirum, were living and allowed 

 to dry in the sun. The next day the mud was thoroughly 

 dried so it would readily crumple between one's fingers. 

 In this condition it was kept until August 10 when the 

 finger-bowl was filled with spring water. On the follow- 

 ing day several individuals of each of the above three 

 species were swimming about in the water. 



When ponds and pools in which rotifers live are in the 

 process of drying up the water becomes so foul by the 

 decomposition of dead plants and animals that all the 

 rotifers of some species die before the pool is completelv 

 dried. If, on the other hand, rotifers are kept in the 

 laboratory in very clean water which is allowed to slowly 



8 Jour. Eccper. Zool., Vol. V, p. — . 



