398 



CHAPTEE XX. 



ARACHNID A, MYRIAPODA, AND INSECT A. 



CLASS ARACHNIDA. 



THE Araclinida are Articulate animals, in which the respira- 

 tory organs, when present, are in the form of pulmonary 

 chambers or sacs, or of ramified tubes (" trachew ") formed by 

 an involution of the integument and fitted for breathing air 

 directly ; or both these organs cure, combined. In no case are 

 the breathing -organs in the form of gills. There are four 

 pairs of locomotive limbs, and there are no limbs attached to 

 the segments of the abdomen. There is only one pair of an- 

 tennce, and these are not present as antennae, but arc converted 

 into jaws or pincers. The head is amalgamated with the 

 thorax to form a ceplialothorax, the eyes are sessile, and the 

 integuments are more or less chitinous. 



The Arachnida are mainly distinguished from the Crus- 

 tacea^- by the absence of gills, and the general presence of 

 organs adapted for breathing air directly. They are distin- 

 guished from the Insects by the possession of four pairs of 

 legs, by never possessing wings, and by having simple eyes, 

 whilst the head is amalgamated with the thorax. From the 

 Myriapods they are distinguished by the fact that the legs 

 of the latter are never less than nine pairs in number, whilst 

 the segments of the thorax are distinct from one another and 



1 Van Beneden would refer the Trilobites, King-crabs, and Eurypterids to 

 the Arachnida, but such a radical change must be supported by overwhelming 

 evidence before it can be accepted. 



