258 ANNULATA. 



In addition, the segments of the thorax are free. They are 

 probably the most primitive of insects. The common silver- 

 fish ( Lepisma) is a good example. 



Class V. — x\kachnida. 

 The spider is not so typical of the Arachnida as is the 

 cockroach of the Insecta. The Arachnida are a more 

 primitive class and the various orders are more divergent 

 in structure than those of the Insecta. 



As a class they are distinguished by the absence of pre- 

 oral appendages or antennae, by the division of the body 

 into cephalothorax and abdomen, or no division. They 

 resemble the insects in the common presence of tracheae, 

 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 chelae (called 

 chelicerce 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 pectines or combs, probably 

 tactile in function, and the next four have diagonal slits on 

 their ventral surface, the stigmata^ leading into the lung- 

 books. The seventh segment has no appendages noi 

 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 ir 

 the possession of simple eyes, coxal glands and the genera 

 structure of its body, but its nervous system is less con 

 centrated. 



Order II. — Araneina, 

 The spiders are a widely distributed and successful orde: 

 of Arachnida. They prey naturally upon insects whicl 



