ARACHNIDA. 151 



CHAPTEE XIV. 



ARACHNIDA. 



Class II. Arachnida. 



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 life, so that when any distinct 

 breathing-organs ai'e 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/ojjr pairs 

 of legs, and the segmerUs of the abdomen never carry locomotive limbs 

 of any sort. The eyes are sessile, and are' never supported upon 

 stalks; the antennce are converted into jaws or 'pincers; and the head 

 is always amalgamated with the thorax, so as to form a cephalothora.r. 



The integument usually produces chitine more or less abundantly, 

 so as to constitute a resistant shell ; but in some cases the skin re- 

 mains 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 "maxillse," 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 (fig. 104, B), perforated by a canal which 

 communicates with a poison-gland situated near its base. By means 

 of this poison apparatus the spiders kill such animals as they 

 capture. In the Scorpions the mandibles are short, and terminate 

 in strong pincers (fig. 101, c). In them, too, the maxillte are fur- 

 nished with enormously developed nipping-claws or chelae. In all 

 the Arachnida the mandibles are believed to correspond to the 

 antennae of the Crustacea. In the lower Arachnida, such as the 

 Ticks, the organs of the mouth are modified partly for piercing, and 

 partly for suction. 



The mouth in the Arachnida opens into a pharynx, which is of 

 very small diameter in the true Spiders, which live simply on the 



