RESPIRATION. 



567 



(Fig. 244). The water is taken in by a process of swallowing, the mouth 

 being first distended, and, as the muscles contract, the water is expelled 

 through the aperture on either side of the pharynx into a cavity called 

 the gill-cavity, and, as it passes over the gills, the oxygen of the atmos- 

 phere held in solution is absorbed by the blood. 



Fish are thus admirably fitted for aquatic respiration, but die on 

 removal from the water from the fact that, as the gills dry, absorption 

 of oxygen is impaired, and the gills cling together, and so prevent ex- 

 posure of their greater portion to the air. Under such circumstances 

 fish then die from asphyxia. In some cases there is provided in addition 

 an air-bladder or swimming-bladder, like a rudimentary sac of the air- 

 breathing apparatus of the higher animals. In its simplest condition 

 it is entirely closed, and can, therefore, serve no purpose except to 



C 



Fig. 242.— Diagrammatic Section of a Lamellibranch (Fresh-water 



Mussel, Anodon) Through the Heart. (Huxley.) 



F, ventricle ; G, auricles; C, rectum; P, pericardium : H, inner gill; I, outer gill; O Q, organ of Bojanus ; 



11, foot; A A, mantle-lobes. 



regulate the specific gravity during swimming. In others this bladder 

 is connected with the alimentary canal by a short, wide tube, called the 

 ductus pneumaticus, and is filled by the process of swallowing. 



If we admit, as seems perfectly reasonable, that the air-bladder of the 

 fish is a rudimentary lung, it ma}' be stated that all vertebrates in the 

 course of their life have two different kinds of respiratory apparatus. 

 Every form of vertebrate breathes through gills during embryonic life; 

 in the fish and a few reptiles the gills are permanent, but in others they 

 disappear, and, while traces of a lung are seen in all vertebrates, they 

 acquire full development only in reptiles, birds, and mammals : while, 

 again, certain amphibians, as the Proteus and Siren, retain both gills and 

 lungs to adult life, and thus form a link between fishes and reptiles. 



