124 OF RESPIRATION. 



foot, which they help to distend, and communicate with the 

 main cavity of the body, supplying it also with liquid ; in 

 echinoderms they pass through the skin, and even through 



260 a. In order fully to appreciate the homologies between the 

 various respiratory apparatus observed in different animals, it is ne- 

 cessary to resort to a strict comparison of the fundamental connec- 

 tions of these organs with the whole system of organization, rather 

 than to the consideration of their special adaptation to the elements 

 in which they live. In Vertebrates, for instance, there are two sets 

 of distinct respiratory organs, more or less developed at different pe- 

 riods of life, or in different groups. All Vertebrates, at first, have 

 gills arising from the sides of the head, and directly supplied with 

 blood from the heart ; but these gills are the essential organs of res- 

 piration only in fishes and some reptiles, and gradually disappear 

 in the higher reptiles, as well as in birds and Mammalia, towards 

 the close of their embryonic growth. Again, all Vertebrates have 

 lungs, opening in or near the head ; but the lungs are fully devel- 

 oped only in Mammalia, birds, and the higher reptiles, in propor- 

 tion as the branchial respiration is reduced ; whilst in fishes the air- 

 bladder constitutes a rudimentary lung. 



260 b. In Articulates, there are also two sorts of respiratory or- 

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

 aquatic, in Crustacea and worms, called gills. But these tracheae and 

 lungs open separately upon the two sides of the body, (air never 

 being admitted through the mouth or nostrils in Articulates ;) the 

 gills are placed in pairs ; those which are like the tracheae occupying 

 a similar position, so that there are nearly as many pairs of tracheae 

 and gills as there are segments in these animals, (Figs. 89 and 33.) 

 The different respiratory organs in Articulates are in reality mere 

 modifications of the same apparatus, as their mode of formation and 

 successive metamorphoses distinctly show, and cannot be compared 

 with either the lungs or gills of Vertebrates ; they are special organs 

 not found in other classes, though they perform the same functions. 

 The same may be said of the gills and lungs of mollusks, which 

 are essentially alike in structure, the lungs of snails and slugs being 

 only a modification of the gills of aquatic mollusks ; but these two 

 kinds of organs differ again in their structure and relations from the 

 tracheae and gills of Articulates, as much as from the lungs and gills 



