G. ORGANS OF RESPIRATION. 



THE respiratory organs are closely connected with the ali- 

 mentary canal, both in position and development, and are of 

 two kinds, gills and lungs. The former, as the phylogenetically 

 older organs, are adapted for aquatic respiration, and are con- 

 nected with the pharynx in the region of the visceral or branchial 

 arches : the latter always arise as sac-like outgrowths of the primi- 

 tive oesophagus, which grow backwards so as to lie within the 

 body-cavity. 



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

 but with few exceptions (Dipnoi, and possibly Siren amongst 

 the Perennibranchiata) they are not functional at the same time. 1 

 Which of these are to serve as respiratory organs depends upon 

 the condition of the circulation, for respiration can only take place 

 where venous trunks, the blood in which is laden with car- 

 bonic acid, come into close relation with the surrounding 

 medium; the carbonic acid is then given off, oxygen being 

 taken up in its place. The venous blood is thus converted into 

 arterial blood, which is again distributed over the body. 



As long as these conditions for the oxidation of the blood are 

 not fulfilled, we cannot speak of a respiratory organ. Thus the 

 air-bladder of Fishes, which arises in a very similar manner to a 

 lung, that is, as an outgrowth of the fore-part of the alimentary 

 tract, has in no period of life the arrangement of the blood-vessels 

 described above: it receives arterial blood only, from the aorta, 

 and venous blood passes from it; it is therefore morpho- 

 logically, but not physiologically, a lung. 



I. GILLS. 



The gills arise as a series of laterally-arranged outgrowths of 

 the throat lying one behind the other, and, in the course of their 

 development, they become open to the exterior. A passage is thus 

 formed for the water entering by the mouth, and in order that 



1 Comp. air-bladder of Lepidosteus, p. 257. 



