52 MARVELS OF FISH LIFE 
line of its body shows the wriggling movement of the 
fish as it endeavoured to escape. 
At first the alevin, exhausted with the exertions of 
escaping from its shell, lies panting on his side, but 
soon he takes up a comfortable position in the gravel, 
and keeps very much to this one spot, until his yolk 
sac is almost absorbed. Alevins are not gregarious, and 
avoid each other; they also dislike the light, and when 
covered with insufficient gravel they burrow deeper 
among the stones. 
I have already alluded to the continual fanning 
movement of the pectoral fin, which commences even 
before the alevin is completely hatched. The result of 
these continual fin movements is to make a current 
round the little fish as he lies buried deep in the gravel, 
thus removing the water vitiated by his breathing. 
When trout are bred artificially, large numbers of 
alevins are kept in hatching boxes, and here the fish, 
when healthy, crowd together or “pack” in the dark 
corners. This packing is due to the instinct which 
makes them seek shelter, and as there is no gravel for 
them to hide in, they hide amongst themselves. An 
incidental result of this packing is, that a steady current 
of water is maintained round the fish, for every alevin 
as he lies shoulder to shoulder with his neighbour, keeps 
up the continual fanning of his two large pectoral fins 
as already explained, so that the polluted water is swept 
on in a steady stream, to be replaced by a fresh oxygen- 
laden supply. 
