A CO-OPERATIVE BREATHING SOCIETY. 59 



clogged with cast-off shells as to make the current sufficiently strong 

 through the remainder to catch and hold any unfortunate alevin 

 passing. It is not safe to heighten the water in the hatching-box 

 by placing flannel on the outlet screen before nine-tenths of the 

 eggs are hatched, or the decreased current will cause partial 

 suffocation of the embryos. And even when one-tenth of the eggs 

 are left, they should be well separated with a feather before the 

 water is deepened. If alevins get against the safety- screen, the 

 pressure of the water gradually forces the yolk-sac, which is very 

 elastic, through one of the perforations of the zinc. If No. 9 size 

 is used, little harm accrues, as a strong alevin, so soon as he feels 

 the process commence, can free himself by his struggles ; and he 

 takes care in future to avoid the screen. But with a smaller size 

 matters are very different. The least portion of the yolk-sac 

 slipping through which it does the more easily, as the smaller the 

 number of zinc the stronger the current bulges out, and keys the 

 fish on the reverse side of the safety-screen in such a manner 

 that escape is impossible. 



At Howietoun the eggs are laid down to hatch between nine 

 and ten in the morning, and, unless the day is very cold, the hatch 

 is completed by three in the afternoon, when the shells are 

 skimmed off and the water in the box heightened. This is tedious 

 work, and we do not care to hatch more than a quarter of a million 

 in one day, although it is occasionally necessary to lay down a 

 much larger number. After the shells are skimmed off and the 

 water heightened in the boxes, the attention required is reduced 

 to a minimum. As the fish get stronger the current is increased, 

 the regulating tap being moved once a week through a space 

 equivalent to 1 quart per minute per box. Hardly any alevins 

 die except in experimental lots. The boxes themselves never 

 require cleaning ; the alevins are their own housemaids. They are 

 constantly scouring over the bottom of the box, and keep the 

 charred wood polished like a piece of dark mahogany. The current 

 carries the dirt through the safety- screen, and it settles in the 

 space of 13 inches between that and the outlet. The rough of 

 this dirt is removed with a syphon every morning. Once a week 



