HOW FISHES BREATHE. 27 



swallowed by this act, because the gullet, just 

 behind the hindmost slit which pierces its sides, 

 is able to contract itself so tightly as to prevent 

 the entrance of any water whatever. This con- 

 traction is performed by muscles, which bring 

 about the same result as the double string run- 

 ning round the mouth of a bag, the which it 

 closes by drawing the mouth smaller and smaller. 

 When the fish desires to swallow food this is 

 pressed against the centre of this closed-up gullet, 

 albeit ever so lightly. The touch signals to the 

 nerves controlling the muscles to relax their 

 hold somewhat, and at the same time to seize 

 upon the newly arrived solid refreshment. This 

 it does so perfectly that little or no water is 

 swallowed therewith. The gullet presses round 

 so tightly that the matter being swallowed 

 might be likened to a cork being thrust down 

 into a bottle with a flexible neck, which closed 

 up as the cork passed lower and lower down. 



This chapter has surely left us in possession 

 of some very interesting facts. Thus we may 

 take it for granted that the gills of fishes were 

 originally formed by delicately waving branches 

 projecting on either side of the head, and that, 

 for protection's sake, they came to be withdrawn 

 into a series of little pockets, communicating 

 with the outer world by a series of slits. The 

 next stage in their history is that in which the 

 walls of these separate gill-pockets or pouches 

 become removed, so that the gills come to lie in 

 a single large cavity, opening by one slit be- 

 hind the head. Lastly, we must remark that 

 about the time when gill-pouches went out of 



