THE ZOOLOGIST. 



THE ENCYSTMENT OF MACROBIOTUS. 



By James Murray (Biologist on the Lake Survey under 



Sir John Murray). 



Many of the smallest animals, in common with some of the 

 largest, experience regularly at some period of the year un- 

 favourable conditions which threaten the destruction of the 

 individual and the extermination of the race. This fate is 

 avoided by one of two means — either the individual perishes, 

 and the race is saved by the production of what are called 

 winter-eggs or resting-eggs, or the individual protects itself in 

 some way, and lies dormant till better times, or, as it is called, 

 hybernates. 



It is usually the cold of winter which constitutes the threaten- 

 ing condition, hence the terms winter-egg and hybernation ; but 

 it may be quite the converse, and it may be the heat or drought 

 of summer which has to be guarded against. Similar means of 

 protection may be used against either evil. Many resting-eggs 

 are laid in summer, and many animals, including Water-Bears 

 and Bdelloid Kotifers, lie dormant during droughts. Using the 

 term in the wider sense to cover all means taken to tide over 

 untoward seasons, large numbers of animals hybernate — Bears, 

 Tortoises, Water-Bears, Bdelloid Botifers, Nematodes, &c. 



Bdelloid Botifers resist desiccation by coating themselves 

 with an air-proof varnish. Water-Bears (or Tardigrada) are 

 equally well protected against desiccation, though the means by 

 which they protect themselves are not understood. There is no 

 coating of varnish in this case. The Bears are found among 

 dry moss, rigidly extended as though in rigor mortis, but they 

 quickly revive when placed in water. 



The process now to be described is of a totally different kind. 

 Its occurrence at the beginning of winter suggests that it is a 

 sort of hybernation ; the profound changes in the organization of 

 the animals which characterize it suggest the name encystment. 



