228 PHYSIOLOGY OF THE INVERTEBRATA. 



The Myriapoda. 



The respiratory organs of these Arthropods are tracheae. 

 The tracheae form a branched series of tubes spreading 

 throughout the body and conveying oxygen to the various 

 organs-, tissues, and blood-vessels. The tracheas communicate 

 with the exterior by openings called stigmata, which are 

 situated on " the lateral or ventral surface of more or fewer 

 of the somites. In Smitigcra * the stigmata are situated in the 

 median dorsal line of the bodj'." 



In this class of animals the function of respiration is of a 

 much higher order than in any other air-breathing animal 

 alluded to in the present chapter. We find in the Myriaiioda 

 a special set of tubes set apart for respiration and by means 

 of these tubes the air is brought into contact with the blood- 

 vessels distributed over the walls of the tubes. Although 

 these animals breathe principally by means of a tracheas or 

 air-tubes, they also breathe in lesser degree by their general 

 surface ; but this kind of respiration is more marked in those 

 animals whose integument is unprotected by epidermal 

 developments. 



The introduction and expulsion of the air in the tracheae 



appear to be helped by regular movements of the abdominal 



walls. 



The Insecta. 



In the Insecta the systems of trachea? or air-tubes are 

 further developed. The ultimate ramifications of these tubes 

 constitute a fine network, analogous in many respects to the 

 capillary networks. 



As in the Myriapoda these trachea communicate externally 



by means of stigmata. These stigmata are restricted to the 



somites of the abdominal region of the bod}'- ; and very 



frequently these openings are occupied by perforated plates. 



The perforated plates act as sieves or filters, and thus free 



the air, as it passes through them, of mechanical impurities. 



* See also Sinclair in Proc. Poy. Soc, 1871 ; and Nature, Dec. 17, 1891, 

 p. 164. 



