ANIMALS— PHYSICAL ENVIRONMENT 95 



extends over years. Recurring periods of drought 

 are necessary for their existence, so that it is not 

 remarkable that they should be found in deserts 

 and semi-deserts in nearly every part of the globe. 

 One member of this group, the Brine Shrimp 

 (Artemia), is not only extremely resistant to drought 

 in the egg stage, but also requires for its development 

 water nearly or quite saturated with salt ; and it is 

 found in temporary salt pools in many parts of the 

 world, both in deserts and other situations. Certain 

 Phyllopods are capable of extremely rapid growth 

 when the mud in which their eggs have been dormant 

 is flooded, and when the shallow pool is heated by a 

 desert sun. Spencer records that Apus in Central 

 Australia reaches a length of 2^ inches in fourteen 

 days after rain falls. 



Other inhabitants of temporary pools descend into 

 the mud when the pool dries up, and there continue 

 to exist. Examples are the crab (Telphusa) and 

 crayfish (Astacopsis) of Central AustraHa, and many 

 water-beetles and molluscs. Of the Gasteropod 

 molluscs many are protected by an operculum, with 

 which to close the mouth of the shell : Isidorella 

 newcombi, in water-holes in Central Australia, plugs 

 the shell with mud, which is probably specially 

 prepared by passage through the ahmentary canal. 



The biology of the frogs found by the Horn 

 Expedition to Central Australia has been treated 

 at some length by Spencer. Among the species 

 collected were some which showed marked abiUty 

 to breed rapidly and take advantage of a short 

 favourable period, and others whose habits enabled 



