278 Hlow to obtain it. 
been badly used. The fins change with age, and it is, there- 
fore, possible that a fish may at different times of its life have 
a forked tail or a square 
tail. 
A tail sometimes ap- 
pears to be forked because 
it is not fully expanded, 
and the accompanying 
diagram (Fig. 46). will 
show how a forked tail 
may sometimes be made 
into a square one, by ex- 
panding a little. 
Many of the older 
writers have defined species by the numbers of the pyloric ceca, 
but these are found on examination to be ever-varying quantities, 
even in forms that have been supposed to constitute distinct species. 
From twenty-five to ninety the number of the former seems to 
range, the average being between forty and fifty. The usual tem- 
perature of a trout is nearly about the same as that of the water 
in which it lives. Red spots may sometimes be seen on trout 
after death that were not visible during life. 
Fig. 46. 
