CLASS CBtJSTACEA. 229 



plies rapidly, and produces the loathsome 'disease to which it 

 . gives name. 



Mq. 391. 



Ixodes 6oBis, Western Cattle Tick. 



Ixodidae. — Ticks infest man and 



beast in great variety. Quickly bury- 



sarcoptes acaiAei, ing themselves beneath the skin, they 



-mite,magni ec. cannot readily be extracted without 



leaving behind portions of their claws, which, if allowed to 



remain, soon produce inflammation and serious ulcers. 



CLASS II. CRUSTACEA. ^ 



General Characteristics. — The Crustaceans (hard- 

 covering) are articulate animals with jointed appendages 

 and essentially aquatic respiration. The body typically con- 

 sists of twenty segments, though some species have sixty 

 and some only ten. The hard exterior crust not yielding 

 for growth, frequent moultings are necessary, which are 

 effected by the shell splitting open, generally on the back, 

 and the animal crawling out. The heart, when it exists, is 

 on the back, immediately under the shell. The nervous 

 system Kes on the under surface. In their growth from the 

 egg they undergo the most astonishing changes, the young 

 in no way resembling the adult. The Crustacea are mostly 

 marine ; some, however, live in fresh water, and a few in 

 damp places on the land. 



