Spiders and Mites 109 



comb-like organs in the scorpion may be such. The 

 parts of each limb are like those in Crustacea; the 

 body and its organs are however much shorter than 

 in that class. The nervous system is concentrated, 

 the digestive canal often has blind pouches appended 

 to it. There is an abdominal heart in all, except in 

 a few mites, and there is usually some kind of breath- 

 ing organ. Organs of touch, smell and hearing seem 

 to be deficient. 



Many of these animals are parasites, either ex- 

 ternal, or internal ; but except in these, there are few 

 in which the young undergo much metamorphosis 

 after hatching. 



The outer surface is often hard and chitinous, but 

 never calcified. The dorsal surface layer is seldom 

 extended over any of the neighbouring segments or 

 appendages, or when extended it is immovable. 



Mites. The three chief orders in the class consist of 

 mites, spiders and scorpions. Of these the mites are the 

 simplest and are exemplified by the cheese- FIG. 60. 

 mite, found in mouldy cheese, or the 

 sugar-mite often met with in brown sugar. 

 In these the abdomen is unsegmented, 

 and usually indistinctly separate from the 

 cephalo-thorax. The breathing organs 

 are fine tubes named tracheae, which open cheese-mite 

 on the surface, and break up within the (Acams). 

 body into branches, which admit air into the tissues. 

 The mouth in mites is often proboscis-like or armed 

 with a spiny beak. Most of them are parasites either 

 upon animals or plants. One curious group inhabits 

 the body cavities of vertebrate animals, wherein their 

 worm-like bodies may be mistaken for tape- or thorn- 



