ARTHROPODA. 193 



I have put a haadful of empty univalve shells, say a 

 dozen, or more, into a basin with a hermit-crab, and 

 watched him try on every one of them, ■ though they 

 were all a misfit, and he had to go back to his own 

 in the end. If you require to be convinced that in 

 the matter of acquiring worldly goods crustacean 

 nature and human nature are identical, study two or 

 three hermit-crabs for a day ; but remember to put 

 them back in the sea afterwards. 



The Aeachnida, or Spiders, are defined as " air- 

 breathing Arthropoda with fused head and thorax, 

 with two pairs of jaws, four paii-s of walking legs, and 

 no feet attached to abdomen." The Mites and Mock- 

 scorpions are nearly related to them. 



The Acwrina, or Mites, are exceedingly simple 

 animals; they have no heart or blood-vessels, and 

 often no eyes, while the nervous system is reduced to 

 one ganglion. Many of them are parasitic, and have 

 a mouth adapted for suckiug, called a Kostrum. 

 Nearly every animal has its own special species of 

 infesting mite. The ticks {Zicodes) are a kind of mite, 

 but larger. The common Cheese-mite [Tijroglypliiis) 

 is a familiar example; it also lives on flour : others live 

 on sugar, etc., and many on plants, which they soon 

 destroy. There are also marine mites. The young of 

 many kinds have at first only six legs, instead of eight. 

 Some of the parasitic forms are curiously altered 

 to suit their mode of life. A mite called Bemodex 

 folliculornm lives in the hair-follicles of animals and 

 man : it has an elongated body and very short legs. 



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