INTERNAL ORGANS 175 



changing the volume of gas is limited, however, and the process 

 of secretion or absorption is by no means always a rapid one. 

 For this reason a sudden rise or fall is dangerous to the fish, 

 and if it ascends suddenly from a considerable depth to the 

 surface it may be quite incapable of descending again. An 

 interesting case of this nature is given by Mr. Semper in his 

 book, Animal Life, concerning one of the White-fishes (Coregonus), 

 an important food-fish of Lake Constance, known locally as the 

 Kilch. He describes how "the fish are caught in nets and 

 brought to the surface of the water; they come up invariably 

 with the belly much distended, the air in the swimming-bladder, 

 being relieved from the pressure of the column of water, has 

 expanded greatly and occasioned this unnatural distension, 

 which renders the fish quite incapable of swimming. Under 

 these conditions the fish is naturally unable to live for any 

 length of time. But the fishermen of the lake have a very 

 simple remedy; they prick into the air-bladder with a very 

 fine needle; the air escapes with some force, the distension 

 subsides, and the fishes are enabled to live under totally changed 

 conditions as to pressure, even in quite shallow water and at 

 the surface. . . . Hence the Kilch is confined to a certain 

 depth, because it is not capable of accommodating the tension 

 of its swimming-bladder to the change of pressure in the column 

 of superincumbent water." A similar experiment has been 

 carried out on fishes in the aquarium, and individuals, floating 

 helplessly at the surface at one moment, have resumed their 

 normal activities as soon as the air-bladder was punctured. 

 Sometimes when deep-sea fishes are brought to the surface by 

 the trawl or dredge, so great is the expansion of the contained 

 gases brought about by the rapid change in external pressure" 

 that the air-bladder is forced out through the gullet and 

 projects from the mouth. Aristotle was aware of this pheno- 

 menon, for he writes that "very often the Synodon and the 

 Channa cast up their stomachs (!) while chasing smaller fishes; 

 for, be it remembered, fishes have their stomachs close to the 

 mouth, and are not furnished with a gullet." 



From the foregoing facts it is clear that a fish like a Shark, 

 which is devoid of an air-bladder, possesses a body which is 

 heavier than the water it displaces and tends to sink continuously, 

 but this downward movement is overcome by a constant 

 movement of the muscular tail and of the fins. Further, in 

 oceanic fishes such as the Myctophids or Lantern-fishes {Mycto- 

 phidae), requiring an exceptional freedom of movement in all 



