CHAPTER XV. 



Aeachnida. 



Class II. Aeachnida (Gr. Arachne, a spider). — This class 

 includes the mites, ticks, scorpions, and spiders, and, as a 

 whole, is very nearly related to the preceding. The Arachnida, 

 however, are distinguished from the Crustacea by being adapted 

 in most cases for a strictly terrestrial Ufe, so that when any 

 distinct breathing-organs are present these are never in the 

 form of gills, but are always either pulmonary sacs or air- 

 tubes {tracheoe). In none of the Arachnida, further, are there 

 ever more than four pairs of legs, and the segments of the ab- 

 domen never carry limbs of any sort. The eyes are always 

 sessile, and never supported upon stalks ; if antennce exist 

 at all, they are much modified, and the head is always amal- 

 gamated with the thorax, so as to form a cephalo-thorax. 



The integument usually produces chitine more or less 

 abundantly, so as to constitute a resistent shell ; but in some 

 cases the skin remains permanently soft. The mouth is situated 

 in the anterior portion of the body, and in the higher forms is 

 furnished with a pair of prehensile jaws, called " mandibles," 

 a pair of chewing-jaws, called " maxUlse, and a lower lip. In 

 the scorpions an upper lip is present as well. In the true 

 spiders each mandible terminates in a sharp movable hook, 

 perforated by a canal which communicates with a poison-gland 

 situated near its base. By means of this poisonous fluid the 

 spiders kill such animals as they capture. In the scorpions 

 the mandibles are short, and terminate in strong pincers. In 

 them, too, the maxillae are furnished with enormously-de- 

 veloped nipping-claws or chelae.* In all the Arachnida the 

 mandibles are believed to correspond to the antennae of the 



* These nippiDg:-claws in the scorpions are produced not by the maxillie themselres, bat 

 by two appendages to the maxillffi, which are known as "maxillary palpi." 



