240 RESPIRATION. 



are fully developed only in mammalia, birds, and the higher 

 reptiles, in proportion as the branchial respiration is reduced; 

 whilst in fishes the air-bladder constitutes a rudimentary lung. 

 405. In the articulata, there are also two sorts of respiratory 

 organs ; aerial, called tracheae in insects, and lungs in spiders; 

 and aquatic, called gills in Crustacea and worms. But the 

 tracheae and lungs open separately upon the two sides of the 

 body (air never being admitted through the mouth or nostrils 

 in the articulata) ; the gills are placed in pairs ; those which are 

 like the tracheae occupying a smilar position, so that there are 

 nearly as many pairs of tracheae and gills as there are seg- 

 ments in these animals. The different respiratory organs in 

 the articulata are in reality mere modifications of the same appa- 

 ratus, as their mode of formation and successive metamor- 

 phoses distinctly show, and cannot be compared with either 

 the lungs or gills of the vertebrata ; they are special organs not 

 found in other classes, though they perform the same func- 

 tions. The same may be said of the gills and lungs of mol- 

 lusca, which are essentially alike in structure, the lungs of 

 snails and slugs being only a modification of the gills of 

 aquatic mollusca ; but these two kinds of organs differ again 

 in their structure and relations from the tracheae and gills of ar- 

 ticulata, as much as from the lungs and gills of vertebrata. 

 In those radiata which are provided with distinct respiratory 

 organs, such as the echinoderms, we find still another type 

 of structure, their gills forming bunches of fringes around the 

 mouth, or rows of minute vesicles along the radiating seg- 

 ments of the body. 



