PARASITES OP ANIMALS. 5 



The head appears to be composed of seven rings, and there 

 are three pairs of mouth organs. They all breathe by means 

 of air tubes or tracheae. 



II. ARACHNIDS. 



The next sub-division, called AracJtmda, includes the spi- 

 ders, scorpions, " daddy-long-legs," or harvest men, mites, etc. 

 In these the head is generally more or less united to, or sunken 

 in, the thorax, as in the spiders, which have the abdomen very 

 distinct; or the abdomen may be closely united to the thorax, 

 Fig. 6. as in the mites (figure 48). When mature they 

 have eight legs. The head bears but few organs. 

 There are no antennas. The eyes are simple, but 

 often numerous. Many breathe by tracheae, but 

 some spiders by lung-like organs, and others by 

 both combined. 



III. MYRIAPODS. 



The lowest sub-elass, called Myriapoda, includes 

 the centipeds, "thousand-legs," etc., (figures 1 

 and 6.) These have the rings of the thorax and 

 abdomen all very similar, and the rings of the ab- 

 domen in some become very numerous, sometimes amounting 

 to several hundred. Each ring usually bears a pair of legs, 

 aiid contains a nerve ganglion. The head is similar to that of 

 the six-legged insects, and bears antennae, and sometimes com- 

 pound eyes. They breathe by air tubes or tracheae. This 

 last sub-class contains no parasites, but in both the others 

 there are numerous examples. Yet in those they are confined 

 to particular orders and families. 



Among the Arachnids we find parasites only in the lowest 

 order, which includes the mites, the true ticks, the itch-insect, 

 etc., but among the Hexapod insects we find that the parasites 

 belong mostly to two of the seven orders into which they are 

 naturally divided. 



The seven orders of six-legged insects are based on many 

 anatomical differences, but they are characterized, also, by 



FIGURE 6. Lithobius Americanus Newport, natural size; color yellowish brown. 

 From Packard's Guide. 



