6 
I have already given you, in the gizzard-shad, one example of a 
young fish especially armed with teeth for this sort of hunting, and I 
may mention, in passing, another case of the kind which is even more 
remarkable. The common whitefish of the Great Lakes is, as you all 
know, quite toothless, and, as an adult, is what the Germans call a Klein- 
tierfresser—a convenient word for which we can hardly substitute its 
literal translation—a small-animal-devourer. The recently hatched white- 
fish, however, is a pure plankton-eater, and it must snatch its minute 
prey, one at a time, from the sparsely inhabited waters of the open lake. 
It is very important to it, consequently, that it should not miss its catch 
or lose its hold, and we find it specially equipped against this accident 
with four acute, curved, raptatorial teeth on its lower jaw (Fig. 7), as 
effective against a Cyclops or a Diaptomus as the fangs of a tiger against 
an antelope. 
Such transformations in food habit with increasing size are, indeed, 
the rule among fresh-water fishes. Starting together as plankton-eaters, 
they presently diverge in habit, reaching their adult food stage through 
two or three degrees of change. The sheepshead (Fig. 8), for example, 
begins, like the rest, with plankton, becomes insectivorous when it is a 
few inches long, living almost wholly on the insect larve of the bottom, 
and as it reaches adult size its habits change again to those of a mollusk- 
eater, in adaptation to which it develops in its throat a powerful crush- 
ing apparatus, with pharyngeal jaws capable of smashing the thickest 
shells of our water snails, and even those of clams or mussels of con- 
siderable size. 
Other mollusk-eaters are the Great Lakes sturgeon (Fig. 9) and 
certain species of the catfish (Fig. 10), sucker (Fig. 11), and sunfish 
families (Fig. 12), several of them especially equipped for crushing 
shells—the suckers and sunfish by stout, blunt teeth set in their strong 
pharyngeal jaws (Fig. 13), and the catfish by pads of sharp conical 
teeth in their premaxillaries (Fig. 14) and mandibles. By the use of 
these they seem able to crack a snail as a boy cracks a hazelnut, reject- 
ing the broken shells to swallow the juicy meats. Among the suckers 
there is a curious inverse correlation in the development of certain of their 
feeding structures, gill-rakers and pharyngeal jaws growing, one may 
say, each at the expense of the other. That is, where gill-rakers are 
long and numerous, the pharyngeal jaws are weak and their teeth are 
numerous and small, and the species feeds largely on Entomostraca; 
while if the pharyngeal jaws are thick and strong, with strong crush- 
