ENCYSTMENT OF TARDIGRADA. 849 



Conclusions. 



What purpose does encystment serve in the life-history of the Tardigrada ? En- 

 cystment, if not accompanied by absorption of organs, might be regarded as simply a 

 sort of hibernation. This view of its meaning is supported, in the case of M. dispar, 

 by the fact that the encystment took place in the beginning of winter, just when the 

 shallow ponds in which the animals live were beginning to skin over with ice on cold 

 nights. 



On the 18th November 1906 the pond was completely frozen over in the morning. 

 At 11.30 a.m. there was open water at one side, having a temperature of 39°"0 F. At 

 the other side the surface was still covered by ice, and the temperature under the ice 

 was 36°'0. In moss taken from under the ice there were many Tardigrada (M. dispar) 

 beginning to encyst. Later in the season, when the pond was frozen nearly to the 

 bottom, the ice was broken and moss adhering to it washed. There were now numbers 

 of cysts, but no active animals. When this moss had been kept in a warm room for 

 a number of hours, active animals began to appear, and it was then that the emergence 

 from the cysts was studied. In the course of a day or two the active animals became 

 very numerous. These facts might indicate that the process is nothing but a hiberna- 

 tion. But the return to a simpler condition puts another aspect on the matter. In 

 what way can it benefit a hibernating animal to absorb its legs and other organs, and 

 afterwards grow a new set of them ? There is surely waste here, while in the familiar 

 instances of hibernation, physiological activity is so low that waste is reduced to a 

 minimum. 



Is there, then, in this absorption and regeneration of parts anything analogous to the 

 rejuvenescence of lower forms ? Does the animal retain its individuality throughout 

 these changes ? 



If Megnin were right in his theory, that the sarcodic plasma formed during the 

 ecdysis of Acarina was enveloped by a veritable blastoderm, the process might be con- 

 sidered a reproductive one. 



In the Tyroglyphidse studied by Michael the gradation which may be traced from 

 species which have an inert, amorphous cyst, similar to that of Tardigrada, to those 

 more closely resembling the nymph, and having rudimentary limbs, makes it clear that 

 the cyst is merely a stage in the development of the individual, and by analogy we may 

 suppose that this is the case with Tardigrada also. 



The ordinary, active hypopi of the Tyroglyphidse are adapted to secure distribution. 

 Megnin suggests that the nymphal skins containing the cysts might be blown about by 

 the wind, but Michael does not see why this should not as readily occur with ordinary 

 inert nymphs before the ecdysis. 



However it may be with Acarina and with Tardigrada living among terrestrial moss, 

 the encystment of M. dispar cannot be supposed to assist distribution. It is an 



