THE ARTHROPODS 379 



of farm animals. They also, in certain cases, carry the germs 

 of dangerous diseases (e.g., Texas fever of cattle). 



The itch-mite which burrows in human skin is a member of 

 this group. The mange of dogs is caused by another mite. 

 The remedy against all the mites and ticks which are skin 

 parasites consists in covering the skin with soap, ointments, 

 or solutions containing sulphur, tobacco, lime, or other sub- 

 stances which are poisonous to these animals. In treating 

 the skin of sheep, cattle, and other farm animals, it is usually 

 most convenient to dip the animals into tanks containing 

 the solutions. Special machines for lifting heavy animals 

 into and out of such a bath have been devised, and sheep 

 in large numbers are often driven through a narrow pathway 

 which leads into a long tank containing hundreds of gallons 

 of the solution through which the animals must plunge. 



MYRIOPODS 



321. Centipedes and Millipedes: Myriopoda. These 

 are worm-like in form, but have the hard exo-skeleton and 

 jointed appendages which are characteristic of arthropods. 

 The segments of the body are similar, and each bears one 

 or two pairs of appendages (one pair in centipedes, two in 

 millipedes). They breathe by means of much-branched air- 

 tubes (tracheae) which open by pores on the surface and 

 ramify through the tissues, thus conducting air directly to 

 the cells. 



Centipedes have poison glands, and the bite of some large 

 tropical species is fatal to small animals and causes great 

 pain to men. The bite is inflicted by the hooked ends of the 

 first feet. The common house-centipede is harmless, and 

 destroys insects (see a special circular of United States De- 

 partment of Agriculture). There is no reason to fear the 

 ordinary centipedes of temperate climates, and the millipedes 

 (" thousand-legs") are also harmless. 



