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THE INDIVIDUAL ORGANISM 



tissues. At their ultimate branches the tracheae communicate with 

 numerous very fine, thin-walled, liquid-filled tubules, the tracheoles, in 

 which by diffusion oxygen is carried to and carbon dioxide from the 

 immediate vicinity of the cells. Since these tubes and tubules are internal 



and comprise a tremendous total linear extent, 

 regular muscular breathing movements are required 

 to bring about the necessary renewal of fresh air. 

 Various devices for external respiration. 

 Except for the insects and their allies, all metazoan 

 adaptations to provide respiratory exchange for 

 internal tissues involve the utilization of some type 

 of circulatory system and the consequent develop- 

 ment of external and internal respiration. The 

 chief variations that occur among these organisms 

 concern the type of respiratory devices that are 

 utilized to accomplish external respiration. 



1. The Skin as an Organ for External Respiration. 

 In the earthworm and numerous other small 

 metazoans, the body surface is thin and moist and 

 has an area sufficient to provide for an adequate 

 gaseous interchange between the blood and the 

 external medium. Here no breathing is required, 

 since the whole body surface is adapted for 

 respiratory exchange. It is worth noting in this 

 connection that the very properties that make 

 the earthworm's body covering a good respiratory 

 membrane make it impossible for the animal to 

 exist in any except moist, protected situations. 



2. Gills as Organs for External Respiration. Gills 

 are special respiratory membranes adapted for 

 a gaseous exchange between the blood 1 and an 

 external aquatic medium. The gills of fishes are 

 broad plates of delicate respiratory tissue richly 

 supplied with blood vessels, attached in the region 



of the pharynx to bony arches; between these are slits through which 

 water taken into the mouth is passed out. Many aquatic animals 

 have gills which, though differing in position and complexity, function 

 on the same general principle as do those of the fish. Examples are to 

 be found among the worms, clams, a few aquatic insect larvae, and 

 tadpoles. 



1 In most aquatic insect larvae, the air of the tracheal system is brought into close 

 proximity to the water in thin delicate outgrowths from the body wall, which are 

 termed tracheal gills. 



Fig. 13.10. The tracheal 

 respiratory system of 

 an insect. Branches 

 from the main trunks 

 and connectives extend 

 to all parts of the body ; 

 their finer ramifications 

 penetrate all tissues 

 and are too small and too 

 numerous to be shown. 

 (From Shull, Principles 

 of Animal Biology.) 



