O. ORGANS OF RESPIRATION. 



THE respiratory organs of Vertebrates are closely connected with 

 the alimentary canal both as regards position and development, and 

 are of two kinds, gills and lungs. 1 The former are phylogenetically 

 the older organs, and are adapted for aquatic respiration : they are 

 connected with the pharynx in the region of the visceral arches. The 

 latter always arise as sac-like outgrowths of the pharynx, which 

 grow backwards so as to lie within the body-cavity. 



Both gills and lungs may be developed in the same individual, 

 but are usually not functional at the same time. They are sup- 

 plied with venous blood, which becomes purified while passing 

 through their capillaries. 



The air-bladder or siuim-bladder present in many Fishes, and 

 acting as a hydrostatic organ (p. 280), arises in a similar manner to 

 the lungs that is, as an outgrowth from the fore-part of the ali- 

 mentary tract : it receives arterial blood from the aorta, and 

 venous blood passes from it ; but in some cases (e.g., Bony Ganoids 

 and a few Teleosts) it may act as an accessory respiratory organ. 



I. GILLS. 



The gills arise in connection with a series of laterally-arranged 

 outgrowths of the pharynx lying one behind the other, which be- 

 come open to the exterior. Passages or clefts are thus formed for 

 the water entering by the mouth, and in order that oxygen may 

 become absorbed, leaf-like or thread-like vascular processes, the 

 gills or branchiae, become developed in the region of each cleft. 

 These are internal or external, according to their position. 2 



Fishes possess gills throughout life ; amongst Amphibians this 



1 The integument (e.g., in Periophthalmus and Amphibia) and the intestine 

 {e.g., in Callichthys, Hypostomos, and Doras) may also take part in respiration. 



2 External gills persist after hatching as functional respiratory organs only in 

 Polypterus, Calamoichthys, Protopterus, and the Amphibia, and in the Anura 

 they are soon replaced by internal branchiae (comp. pp. 278 280). 



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