PARASITES OF 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 trachee. . 
II.—ARACHNIDS. 
The next sub-division, called Arachnida, 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 antenne. The eyes are simple, but’ 
often numerous. Many breathe by trachez, but 
some spiders by lung-like organs, and others by 
both combined. 
\ 
II.—Myrtapops. 
The lowest sub-class, 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, 
and contains a nerve ganglion. The head is similar to that of 
the six-legged insects, and bears antenne, and sometimes com- 
pound eyes. They breathe by air tubes or trachea. 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. 
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