x RESPIRATORY ORGANS 291 
hyoid arch, and is apparently the representative of the artery 
supplying the hyoidean hemibranch in Elasmobranchs. 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 FE. braziliensis, 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 Protopteruvs, 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 
1 Semon, Zoologische Forschungsreisen in Australien, Pt. i. p. 44, and Atlas. 
2 Burt G. Wilder, Proc. Amer. Ass. Adv. Sci. 1875, p. 151; ibid. 1877, p. 306. 
3 Ann. d. Sci. Nat. sér. 6, vii. 1878, Art. 5. 
4 Baldwin Spencer, op. cit. p. 3. 
