ADAPTATIONS 385 



and the large dental plates, and there is no reason to believe 

 that the surviving forms have undergone a great change in the 

 structure or function of the air-bladder. The most ancient 

 lung-fishes were in some respects more similar to the fringe- 

 finned ganoids (Crossopterygii) than their living descendants, 

 and we have seen that the living Crossopterygii also use the 

 air-bladder for respiration. It seems therefore the most pro- 

 bable hypothesis that this was its original function in these 

 most ancient known bony fishes, and the reason for its evolution. 

 No trace of an air-bladder is present in fishes of the shark type 

 (Elasmobranchii) or in Cyclostomes (Lamprey, etc.), both 

 more primitive types than the bony fishes (Teleostomi). In 

 fact, we may suppose that the original fishes, more or less 

 similar to sharks and dog-fishes, were inhabitants of the sea, 

 where the water being saturated with oxygen there was no need 

 of any atmospheric respiration to supplement the action of the 

 gills. Some of these original fishes ascended the rivers and 

 became inhabitants of fresh water, as some selachians and 

 rays ascend the Amazons at the present day. Some of these 

 forms made their way into streams or lakes where, from the 

 hot climate and the decomposition of vegetable matter, oxygen 

 was deficient and they began to swallow air at the surface to 

 compensate for the failure of aquatic respiration. This appears 

 from the evidence to have been the origin of the air-bladder, 

 and at the same time, of the Teleostome type. From these 

 air-breathing Teleostomes at a very early stage arose the earliest 

 Amphibia, and on the other hand they multiplied in all the 

 fresh waters, until some of them again reached the sea, where 

 the air-bladder lost entirely its respiratory function and became 

 a swim-bladder. 



Although, however, it is probable that the air-bladder was 

 primitively respiratory, it is by no means easy to explain its 

 mode of origin. This question is intimately connected with 

 the relations of the blood-vessels, arteries and veins, of the air- 

 bladder to the heart, and these will therefore be described in 

 discussing the question. The mode of origin which naturally 

 suggests itself first is that the paired air-bladders were modifica- 

 tions of a pair of posterior gill-sacs of the kind seen in Elasmo- 

 branchs. This view was proposed in connection with the 

 evolution of Amphibia as long ago as 1875; it assumes of 

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