436 



ANIMAL RESPIRATION 



of the breathing-pores of a Millipede opens into a small air- 

 cavity, from which a bundle of branching air-tubes penetrates 

 into the adjacent parts of the body in a much more thorough 

 way than in Peripatus. These air-tubes are also much more 

 definite in nature than in the last-named animal, and each of 

 them has a firm elastic lining possessing a spiral thickening, 

 the object of which is to give flexibility and prevent collapse of 

 the walls of the tube. Indistinct traces of such a thickening are 

 seen even in Peripatus. The same end is served in fire-hose 

 and the like by insertion of a spiral wire into the cavity of a 

 flexible tube, so that the flow of water may not be interrupted 

 by any accidental kink. The object in the air-tube is of course 

 to secure a continuous passage of air. 



Centipedes are much more active creatures than Millipedes, 

 and therefore require more perfect arrangements for purification 

 of the blood. The breathing-pores open as before along the 

 sides of the body, where the body wall is much softer than it 

 is above and below, in which regions the skin is covered by a 

 strong horny layer. The pores are less numerous and more 

 specialized than in a Millipede, and the air- tubes which are 

 connected with them ramify through the body in a more thorough 

 way. The bunches of air-tubes are also more or less united by 



connecting tubes, so as to form a con- 

 tinuous system, and this considerably 

 promotes the circulation of air. 



The breathing organs of the greatly 

 specialized Shield-bearing Centipede 

 [Sciciigerd), a very active long-legged 

 creature, with a short body, differ 

 greatly from those found in ordinary 

 Millipedes and Centipedes. There 

 are eight shield -like scales on the 

 upper side of the body, and at the 

 hinder end of each of these (except 

 the last) is a slit which leads into what 

 may perhaps be called a btug-sac (fig. 556). This is a flattened 

 bag suspended in a blood-space, and giving rise on either side 

 to some 300 branched air-tubes, separated only by their thin walls 

 from the surrounding blood, which it is their office to purify. 

 The arrangement is specially interesting, because it resembles 



Fig. 556. — Brcitiiiiig Organs of Shield-bearing 

 Centipede [Scntigera] 



A, Two of the shields, sHghtly enlarged, 

 showing breathing-pores (st.st.). b and c, A 

 lung-sac from above and in vertical section, con- 

 siderably enlarged — the blood-space surround- 

 ing the sac is represented in black. 



