MARINE VEETEBEATE8 317 



from the crustaceans frequently shows itself more in the skin 

 of the fish than in the flesh, as observed in the common red 

 gurnard. 



Most fishes possess a membranous bag containing air, situated 

 just below the backbone, and known as the air-bladder ; but this 

 organ does not exist in sharks and rays and in some of the heavier 

 bony fishes that live on the bottom. The air-bladder is capable of 

 being compressed by the action of certain muscles, and its principal 

 use seems to be the adjustment of the specific gravity of the fish to 

 that of the surrounding water ; but it is interesting to note that the 

 development of this air-bladder is precisely the same as that of the 

 lungs of air-breathing animals, and that in some fishes which live 

 in foul muddy waters it is really a functional lung by means of 

 which the fishes can breathe direct from the atmosphere. 



"We can find space to refer only to one other internal structure 

 of the fish, namely, the roe of the female. This usually consists of 

 a very large number of eggs of small size, sometimes numbering 

 many thousands, and even millions, in a single individual. So 

 numerous, indeed, are the eggs, that were it not for the multitudes 

 of carnivorous animals that devour both eggs and fry, the sea 

 and fresh-water lakes and rivers would soon become so thickly 

 populated that the fish would die in millions for lack of food and 

 air. 



In some cases, however, the eggs are much larger and fewer in 

 number, but these are generally protected from the ravages of 

 predaceous species by a hard covering, as we shall observe in the 

 sharks and rays. 



Finally, a word or two must be said about the distribution of 

 fishes. We have already referred briefly to species that live princi- 

 pally at the surface, and others that make the bottom their home : 

 but some of the former go to the bottom for food or to deposit their 

 spawn, while some of the latter occasionally rise to the surface and 

 swim in shoals. We have noticed, too, that the paired fins of 

 bottom fishes are sometimes modified into feelers, or into fingerlike 

 processes adapted for creeping. Similar organs, employed un- 

 doubtedly as organs of touch, and called barbels or barbules, are 

 often developed on the chins or jaws of these fishes. 



Although we have to deal principally with the species that 

 belong more or less to the shore the littoral fishes we should 

 like to refer briefly to one or two interesting features of those that 

 live at great depths. It will be readily understood that much light 



