AEACHNIDA. 153 



ture and function to the breathing ■ tulje.3 of the Miiriapixlu and 

 Insecta, and consist of tubes, which cjpen on tlie surface of the body 

 by distinct apertures called " spiracles," or " stigmata," and ramify 

 through tlie tissues internall}'. The walls of the tubes are ]ii-evented 

 from coUajising by means of a spirally coiled thread or filament of 

 chitine, which is wound round their walls within their inner lining. 

 The jiulmonary sacs which occur in the Araclinida are simple cham- 

 bers formed by an inversion of the skin, which constitutes a numbtr 

 of closely set plates or folds. The whole of the interior of the pul- 

 monary sacs is richly supplied with blood, and air is admitted Ijy 

 means of external openings (fig. 101, C). 



The nervous system is of the regular Articulate type, Ijut the 

 ganglia of the ventral chain are often niasseil togetlier in ]:nrticulai- 

 situations. In no case ax"e compound eyei present; ami when dis- 

 tinct organs of vision exist, these arc i:i the form of from two to 

 eight or more simple eyes. 



ORDERS OF THE AKACHNIDA. 

 Order I. Podosomata. 



In this order are included the "Sea-,spideis " {l')i<:)ui<jiiioiin^ Xyra- 

 phon, &c.), which are wholly marine, and aie often j-egarded as re- 

 ferable to the Crustacea. They possess four jjairs <.>f legs, and upon 

 this groiind they have been generally placed in the Araclinida. In 

 some foinis the legs attain an extraordinary length, and contain 

 prolongations from the stomach. They are all grotesque- looking 

 animals, found at low water upon stones or in.-irine plants, or jjaia- 

 sitically attached to marine animals. 



Order II. Acarixa. 



The most familiar members of this order are the Mites and Ticks 

 (fig. 102). They are distinguished by the fact that the abdomen is 

 amalgamated with the cephalothorax to form a single mass. Respi- 

 ration is effected by the general surface of the body or by air-tubes 

 (trachete). The mouth-organs are usually adapted for piercing aud 

 for suction (fig. 102, C), but some Mites have biting mandibles. 



The habits of the Mites aie extremely varied. Sojne are found 

 upon different plants ; others are parasitic upon water-insects when 

 young, but swim :;l)0ut freely when adult ; others are parasitic upon 

 other animals, such as sheep, dogs, insects, &c. ; and others inhabit 

 decaying provisions, as is the case with the well-known "Cheese- 

 mite" {Ac'-irus doiHcslicHs). Two species liave a considerable medical 



