386 FISHES 



course that the gill-sacs are lateral outgrowths of the pharynx, 

 which open by a superficial opening to the exterior. Goette, 

 however, who proposed this theory, maintains that such 

 pharyngeal gill-sacs, lined by hypoblast, i.e. by a continuation 

 of the cells lining the pharynx, occur only in Cyclostomes, e.g. 

 the lamprey, and in Amphibia, while the gill-sacs of Elasmo- 

 branchs are lined by a continuation of the layer of cells cover- 

 ing the external skin. In Teleostomes the gill-septa being 

 reduced to narrow bars, there are no pharyngeal gill-sacs. 

 Goette, therefore, supposes that the paired air-bladder arose in 

 primitive fishes, and that the Elasmobranchs were derived from 

 these before this structure had been evolved. This is a view 

 which other zoologists do not accept, for there seems no reason 

 to doubt that the gill-sacs of Elasmobranchs are outgrowths of 

 the pharynx. 



There is little evidence, however, either in the development 

 or in the blood-supply of the most primitive air-bladders to 

 support the view that these structures are modified gill-sacs 

 which lost their opening to the exterior. If this view were 

 correct we should expect that in an early stage of development 

 at least the air-bladder of each side would receive an afferent 

 branchial artery on the ventral side, and that its efferent vessel 

 would be an efferent vessel on the dorsal side. There is no 

 such arrangement known. In the Dipnoi, in Polypterus, and 

 in Amia the air-bladder receives on each side an artery which is 

 a branch from the fourth efferent branchial artery (epibranchial) 

 as it does in the development of the lungs in terrestrial verte- 

 brata. In Dipnoi the veins from the bladder open into the left 

 auricle of the heart which is almost completely separated from 

 the right, as in Amphibia: the connection is, 'therefore, behind 

 the ventricle, not, as in gill-sacs, in front of it. In Polypterus 

 the veins from the air-bladder join the hepatic veins, that is 

 between the liver and the heart ; in all other bony fishes they 

 open for the most part into the hepatic portal vein, behind the 

 liver, while some of them join the posterior cardinal veins ; the 

 arteries are derived from the dorsal aorta. 



It has recently been denied that even in Amphibia the 

 lungs in development correspond to gill-sacs, and stated that 

 they arise as paired outgrowths in a more ventral position. 

 In Polypterus whose development has recently been studied, 



