AEACHNIDA, 153 



ture and function to the breathing - tubes of the Myriapoda and 

 Insecta, and consist of tubes, which open on the surface of the body 

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

 through the tissues internally. The walls of the tubes are prevented 

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

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

 The pulmonary sacs which occur in the Arachnvda are simple cham- 

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

 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 by 

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



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

 ganglia of the ventral chain are often massed together in particular 

 situations. In no case are compound eyes present ; and when dis- 

 tinct organs of vision exist, these are in the form of from two to 

 eight or more simple eyes. 



ORDERS OF THE ARACHNIDA. 

 Order I. Podosomata. 



In this order are included the " Sea-spiders " {Pycnogonum, Kym- 

 phon, &c.), which are wholly marine, and are often regarded as re- 

 ferable to the Crustacea. They possess four pairs of legs, and upon 

 this ground they have been generally placed in the Arachnida. In 

 some forms 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 marine plants, or para- 

 sitically attached to marine animals. 



Order II. Acarina. 



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 eff'ected by the general surface of the body or by air-tubes 

 (trachese). The mouth-organs are usually adapted for piercing and 

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



The habits of the Mites ai'e extremely varied. Some are found 

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

 young, but swim about 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 " {Acarus domesticus). Two species have a considerable medical 



