X RESPIRATORY ORGANS 29 1 



hyoid arch, and is apparently the representative of the artery 

 supplying the hyoidean hemibranch in Elasmobranohs. The 

 efferent vessel of each gill joins the common trunk formed by the 

 union of the efferent vessels of the normal gills of the same side. 



The cutaneous gills of the Dipnoid Protopterus may also be 

 included in the category of larval breathing organs. They 

 consist of three simple unbranched filaments on each side of the 

 head, and, as in Polypterus, are situated at the dorsal extremity 

 of the external gill aperture (Fig. 309). Although usually 

 represented in the relatively young or half grown specimens 

 ■which, so far, have reached Europe, it is extremely probable 

 that these organs atrophy in older individuals. Similar gills are 

 present in the larval Lepidosiren (Fig. 311), but disappear at a 

 much earlier stage. At no period of its development are larval 

 gills present in Neoceratodus} 



The Air -Bladder as a respiratory Organ. — In certain 

 Fishes the air-bladder may become subservient to the function of 

 respiration. In Amia and Lepidosteus the internally sacculated 

 and vascular air-bladder is obviously adapted for air-breathing, 

 and there are not wanting observations ^ which suggest that the 

 organ is actually used for this purpose after the fashion of a 

 lung. According to Jobert,^ this is also the case with the 

 sacculated air-bladder of certain Brazilian Teleosts, viz. Sudis 

 gigas, Erythrinus taeniatus and K hraziliensis, since these Fishes 

 die of asphyxia when the organ is cut off from communication 

 with the exterior by the ligature of its ductus pneumaticus. It 

 is in the Dipnoi, however, that the air-bladder becomes most 

 completely a true lung. In Neoceratodus * the lung is probably of 

 the greatest use to the Fish when the rivers are low during the hot 

 season and the water is charged with foul gases from decompos- 

 ing vegetable matter, and possibly also when the water is filled 

 with sediment in the rainy season. In Protopterus, and more 

 especially in Lepidosiren, the partial atrophy of the gills renders 

 it highly probable that the lungs are the principal breathing 

 organs at all times. Nevertheless, it must be emphasised that 

 in all these Fishes respiration by means of the air-bladder 



' Semon, Zoologische Forschungsreisen in Australien, Pt. i. p. 44, and Atlas. 

 ' Burt G. Wilder, Proc. Amer. Ass. Adv. Sci. 1875, p. 151 ; ibid. 1877, p. 306. 

 ' Ann. d. Sci. Nat. s&. 6, vii. 1878, Art. 5. 

 * Baldwin Spencer, op. cit. p. 3. 



