BACKBONELESS ANIMALS WHICH BREATHE IN AIR 437 



to some extent the breathing apparatus which is characteristic of 

 Scorpions, of which details will be given later. 



It is noteworthy that in animals which breathe by air- tubes 

 the blood-system is in a comparatively ill-developed condition, 

 although the other organs of the body may be exceedingly com- 

 plex and specialized. For while irregular blood-spaces of different 

 size are found in all parts of the body, there is not, as one might 

 perhaps expect, an elaborate set of blood-vessels, but only a 

 heart and, it may be, some exceedingly delicate arteries which 

 soon merge into the blood-spaces. The heart is often called 

 the dorsal vessel, beingf a longr thin-walled tube running near the 

 upper surface of the body. Its sides are provided with numerous 

 pairs of valvular openings through which blood enters. The 

 imperfect state of the blood-system is related to the exceptional 

 nature of the means by which respiration is effected. In, say, 

 a fish one great use of the heart and vessels is to pump blood 

 to the breathing organs for purification, but this arduous kind 

 of work is unnecessary in a centipede or insect, for the air- 

 tubes of these creatures carry pure air to all parts of the body, 

 so that the blood of any organ gets rid almost immediately of 

 any carbonic acid gas which it may have evolved, and at the 

 same time the corresponding loss of oxygen is made good. The 

 organs of circulation are therefore relieved of a large amount 

 of work, and the chief duty which remains to them is that of 

 carrying nutritive material through the body for repair of waste 

 and promotion of growth, a duty which can be carried out 

 sufficiently well without an elaborate system of blood-vessels. 



INSECTS (INSECTA) AS AIR-BREATHERS 



In the case of typical insects the breathing-pores are com- 

 paratively few in number, and open into an exceedingly complex 

 system of air-tubes, which permeate all parts of the body. The 

 arrangement has already been briefly described for the Cock- 

 roach (see vol. i, p. 348), in which each of the last two segments 

 of the thorax and first eight segments of the abdomen bears a 

 pair of pores — ten pairs in all. The arrangement of some of 

 the larger air- tubes will be gathered from fig. 557. The internal 

 organs have a silvery appearance when immersed in water, and 

 this is caused by the air contained in the minute breathing- 



