GENERAL FACTS ABOUT SEA FISH 23 



The air bladder, or swimming bladder, the only internal 



organ with which the present volume need concern itself, is 



The Air- i^ ^ measure comparable to the lung in the higher 



bladder, or vertebrates : and in fact in the so-called " lung-fishes," 



Swimming _ ' _ _ , . . 



Bladder, which have living representatives in three continents, 

 the " lung " is a special modification of the air-bladder. 

 Whether to some extent this organ may be regarded as having 

 the value of a lung in fishes that respire with the aid of gills 

 is an open question. Indeed, the functions of this organ are 

 variously estimated. Its connection in some fishes with the 

 sense of hearing has received passing allusion. It is also 

 associated with the only recognised " voice " in fish, little more 

 than a grunting, such as might be produced by the expulsion 

 of air. Matthias Dunn's view, that the " singing bubbles " 

 sent to the surface by pilchards are an effort of voice on the 

 part of those fish,* has not been confirmed. 



Undoubtedly, however, one of its chief uses to the fish 

 must be as a balancer, adjusting its specific gravity, enabling it 

 without further effort to rise or sink in the water, the fish 

 being, in fact, able to change its volume according to the depth 

 and pressure. Many fishes [e.g. all flat-fish) that lack the air- 

 bladder in the adult stage possess it in the larval form. 



The food of fishes is a subject that will necessarily be 

 reverted to in the chapters that follow. Something has already 

 The Food been said on the species that find their food by scent 

 of Fishes, and those which seek it by the aid of their eyesight, 

 and it will generally be found that the latter devour living 

 prey. The very minute foodstuffs on which many fishes feed, 

 either in the larval stage or throughout life, often escape our 

 notice. It is related that the German naturalist Meyer all 

 but lost a number of larval herrings from starvation, owing to 

 the water being admitted to their tank through a strainer. This 

 anecdote will give some idea of the exceedingly small size ot 

 the organisms on which the young herrings must feed, for as 



• See Records of the Falmouth Polytechnic Society for 1892. 



