124 ^^^ L^i^ Story of the Fish 



Devonian drought set in. They must have branched off from 

 the other fishes before the fish which gave rise to both hu- 

 mans and present fishes developed. We do not descend from 

 sharks. 



And this is a scientific fairy story which many scientists 

 believe. 



The air-bladder is, as its name indicates, a bladder. It lies 

 between the stomach and the backbone. In the trouts it is a 

 fragile organ, noticeable only in the inflated condition it as- 

 sumes when the fish has been dwelling in deep water. In 

 shallow-living individuals, it appears to be nothing more 

 than a space inclosed at the top and sides by the inner body 

 wall, at the bottom by a thin shining membrane which the 

 angler's thumb-nail punctures when it pokes in to remove the 

 blood from under the backbone in cleaning the fish. In other 

 species, like the Lower California "sea-bass" {Totuava) 

 whose air-bladder the Chinese esteem highly as a soup-base, 

 it is a conspicuous sac with thick walls which shut it off from 

 the other organs. In all the soft-rayed fishes — the salmon, 

 trout, tarpon, herring, pike, pickerel — a tube leads from the 

 air-bladder to the gullet. This is the tube through which 

 their ancestors breathed air, and because they still retain 

 it they are called primitive — near to the first fishes. In the 

 other fishes the tube no longer exists, and they are called 

 advanced. 



What the air-bladder used to do is pretty well known. 

 It saved the fish from perishing by acting as an auxiliary 

 air-breathing organ. What it does now is not altogether 

 certain. In some fish it serves as an auxiliary sense-organ, 

 in others it is purely part of the internal workings. This is 

 why we have given it a chapter to itself between the two. 



The scientists of the old guesswork school had a theory 

 which seemed to solve the problem. They believed that the 

 purpose of the air-bladder was to act as a hydrostatic sta- 



