Fish and Their Fabulous Neighbors [165 



THE SWIM BLADDER 



The ability of some fish to stay quietly in one place is due 

 to a unique organ known as the "swim bladder." It is in the 

 ibrward part of the body and is filled with gas a mixture of 

 oxygen and nitrogen. Most fish (including the goldfish) that 

 have skeletons of true bone possess this organ; they need very 

 little fin movement to stay at a given depth. When they die, their 

 bodies rise to the surface. 



Sharks and some other kinds of fish have skeletons of gristle. 

 They lack the swim bladder and consequently they can remain 

 suspended in one position only by continuous muscular effort. 

 When these fish die, their bodies sink instead of rising. 



How Fish Breathe 



A fish has very small nostrils which you can see if you look 

 closely on either side of its snout. The nostrils lead to a little 

 sac where the sense of smell is located but they have no connec- 

 tion whatever with breathing. When you see a goldfish constantly 

 opening and closing its mouth, it is seeking air or oxygen and 

 not food. Though they live in water, fish need oxygen just as 

 people do; but they need it in a different form. Most fish "drown" 

 in the air just as a human being will under water if he is sub- 

 merged too long. 



How THE GILLS WORK 



Instead of breathing through nostrils and lungs, a fish is 

 equipped to breathe with gills. You can see its gill covers flat, 

 bony flaps just back of its head, one on each side. When the fish 

 opens its mouth, allowing water to flow in, the gill covers are 

 pressed against the body so that water will not enter from behind 

 them. 



Then you see the gill covers move outward as the water is 

 forced out through the gill slits when the mouth closes. As the 

 water passes through, the oxygen that it contains is absorbed by 

 the tiny blood vessels making up the gills. (At the same time 

 these blood vessels give off carbon dioxide and other body wastes.) 

 The new oxygen thus obtained is then circulated through the 

 body. 



