104 MARVELS OF FISH LIFE 



fectly perpendicular position with their heads in con- 

 fact with the surface of the water. 



During this time, just above the centre of the yolk 

 sac, a number of dark-coloured cells have massed 

 together in a space where the gas bladder is going to 

 appear. These dark cells then draw upwards towards 

 the top of this space, leaving the rest of it clear. Sud- 

 denly, in this space will appear a minute gas bubble 

 caused by the action of these cells on the blood of the 

 fish, which rapidly increasing in size, gives the fish the 

 appearance of having a pearl in his body. The larva 

 now has a primitive gas bladder filled with a bubble of 

 oxygen, and immediately his whole mode of life alters. 

 He now never goes down to the bottom, but floats in 

 the water from six inches to a foot below the surface. 

 The manner of progression also changes, and the young 

 fish advances with a peculiar jerky movement like a 

 water flea, alternately stopping and shooting forward, 

 and at night he rests floating in a horizontal position. 

 All these changes occur in two to three days. 



The larvae henceforward, like the parent fish, swim 

 in shoals, keeping in the sunlight, and carefully avoid- 

 ing dark corners. The perception of these little creatures 

 is extraordinary, and once the gas bladder has appeared 

 they are extremely difficult to catch, persistently dodg- 

 ing the end of the pipette used for picking them out of 

 the water. Even when the pipette is held quite still in 

 the hope that one will come sufficiently near to be 

 sucked up, they give it a wide berth. 



