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TEXT-BOOK OF ZOOLOGY 



according to the parallelogram of forces, into two component forces, of 

 which one (b) is ineffectual, whilst the other (c) impinges on the fin in a 

 vertical direction, and thus raises the fore part of the fish's body. On the 

 other hand, if the fish wishes to sink, it places its pectoral fins in a 

 reverse position, viz., obliquely forwards (illustrate this for yourself by 

 an appropriate drawing). Finally, the pectoral and ventral fins also aid 

 the fish in maintaining its equilibrium ; this is proved by cutting off the 

 pectoral, or both the pectoral and ventral fin of one side of the body, 

 when the fish will turn over to this side, since it now, as it were, forces 

 the body over to this side by means of the two still remaining fins. 

 An organ playing an important part in locomotion is the 

 3. Air-bladder. An air-bladder is present in most fishes (see illus- 

 tration, p. 274). It consists of a membranous sac placed beneath the 

 vertebral column above the abdominal cavity, and is either completely 

 closed or communicates with the digestive canal by means of a passage, 

 the so-called air-duct (ductus pneumaticus). It consists of a single sac 

 or of several segments (two in the carp family), and is always inflated 

 with air, which is secreted from bloodvessels. 



It is somewhat difficult to understand completely the action of the 

 air-bladder. There can be no doubt that its presence reduces the specific 

 gravity of the body of the fish, so that it is nearly the same as that of 

 the water, and has little tendency either to rise or sink. If the air- 

 bladder were surrounded by rigid walls, like a cask or a hollow glass 

 ball, the bulk of the fish would remain the same at any depth, and 

 therefore its specific gravity would remain the same. But the air- 

 bladder and the body walls of the fish are not rigid, and therefore the 

 size of the air-bladder varies with the pressure of the water. The 

 pressure of the water increases with the depth, and supposing the fish 

 lifeless, when it was placed at a greater depth the air-bladder would be 

 reduced, therefore the bulk of the body would be decreased and the 

 specific gravity increased. Thus the fish would begin to sink, and the 

 further it sank the more its specific gravity would increase. Conversely, 

 if the lifeless fish were taken to a higher level, the air-bladder would 

 expand and the fish become specifically lighter. The fish would thus 

 begin to rise, and the higher it rose the more its specific gravity would 

 decrease. The lifeless fish is therefore in the same condition as the 

 hydrostatic apparatus known as the Cartesian diver, which is merely a 

 little figure containing an air-bladder. The figure sinks in water when 

 the pressure is increased, rises when it is decreased. This explains why, 

 if a fish be suddenly brought to the surface from a considerable depth, 

 the air-bladder expands so much that it projects through the mouth, 

 driving the gullet before it, and the fish is unable to get below the 

 surface again. 



