258 ANNULATA. 
In addition, the segments of the thorax are free. They are 
probably the most primitive of insects. The common silver- 
fish (Zepisma) is a good example. 
Cuiass V.—ARACHNIDA. 
The spider is not so typical of the Arachnida as is the 
cockroach of the Znsecta. The Arachnida are a more 
primitive class and the various orders are more divergent 
in structure than those of the Jwsecta. 
As a class they are distinguished by the absence of pre- 
oral appendages or antennz, by the division of the body 
into cephalothorax and abdomen, or no division. They 
resemble the insects in the common presence of trachez, 
in the malpighian excretory organs, and in absence of 
appendages on the abdomen. The four pairs of walking 
legs are usual and the presence of coxal glands in several of 
the orders is important. 
Of the many and divergent orders we can here only refer 
to three. 
ORDER I.—Scorpionida. 
The scorpions are large arachnids. They have six 
pairs of appendages on the cephalothorax, as in spiders, 
but the first two pairs form small and large chelez (called 
chelicere and pedipalpi) respectively, the other four being 
the walking legs. The abdomen is segmented, the first 
seven segments being much larger than the last five. The 
sternon of the first segment has a pair of genital apertures. 
The second bears a pair of fectines or combs, probably 
tactile in function, and the next four have diagonal slits on 
their ventral surface, the s#/gmata, leading into the lung- 
books. The seventh segment has no appendages nor 
apertures. The five last are elongated and form the tail, 
terminating in a post-anal spine. At the base of the spine 
is a poison-gland, a duct from which passes up a groove 
along the sting The scorpion agrees with the spider in 
the possession of simple eyes, coxal glands and the general 
structure of its body, but its nervous system is less con- 
centrated. 
ORDER II.—Araneina. 
The spiders are a widely distributed and successful order 
of Arachnida. They prey naturally upon insects which 
