Why is the perch better armed against the attacks of its enemies than most fresh-water fishes?

Because its spines, when it attains any considerable

PART VIII. F size, protect it from the voracity of other fishes; and when full-grown, even the pike dares not attack it, though the very young perches are its favourite food. Salamanders, small vipers, and young frogs, serve as food to the perch; and M. de Lacepede has assured Baron Cuvier that they seize even young water-rats.

Several species of water-birds, however, pursue the perch with great avidity. It fears thunder, is afraid of frost and ice, and has internal enemies in intestinal worms ; of which, according to Rudolphi, no less than seven species are found in the body of the perch. This fish is very tenacious of life, and Pennant asserts that it may be carried in dry straw for sixty miles without much danger. - Cuvier.

Why is the stickleback so fatal to the perch?

Because the former often erects its sharp dorsal spines at the moment the perch is about to swallow it, which stick in the palate or throat. - Cuvier.