ARACHNIDA. l8l 



CHAPTER XVII. 



ARACHNIDA, MYRIAPODA, AND INSECT A. 

 CLASS ARACHNIDA. 



THE Arachnida are Articulate animals, in which the respiratory 

 organs, when present, are in the form of pulmonary chambers or 

 sacs, or of ramified tubes (" tracheae ") formed by an involution 

 of the integument and fitted for breathing air directly; or both 

 these organs are 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 antenna, and these are not present as antenna, 

 but are converted into Jaws or pincers. The head is amalgamated 

 with the thorax to form a cephalothorax, the eyes are sessile, and 

 the integuments are more or less chitinous. 



The Arachnida are mainly distinguished from the Crustacea* 

 by the absence of gills, and the general presence of organs 

 adapted for breathing air directly. They are distinguished 

 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 



Fig. 124. A recent Scorpion (reduced). The great nipping-claws of the Scorpion are not 

 legs, but are a development of organs belonging to the mouth. 



never less than nine pairs in number, whilst the segments of 

 the thorax are distinct from one another and from the head, 



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

 to the Arachnida, but such a radical change must be supported by over- 

 whelming evidence before it can be accepted. 



