5 
with mud; but it develops effective gill-rakers also, and we sometimes 
find the stomach of the adult stuffed with a fairly clean plankton. 
The stone-roller, although a mud-eater and especially equipped for 
that function by its long intestine, wound in a close coil around the air- 
bladder, nevertheless avoids muddy waters as a rule, preferring quick 
currents over rocky streams, from the stones of which it nibbles and sucks 
the sediment and slime. 
The most remarkable in many ways of our American fresh-water 
fishes is the Polyodon or paddle-fish (Fig. 6), and in nothing is it more 
peculiar than in the fact that, although it is one of our largest fishes, 
reaching a maximum length of six feet and a weight of a hundred and 
sixty pounds, it is essentially a plankton-eater, feeding largely, and some- 
times almost wholly, on the smallest aquatic animals and plants, for the 
appropriation of which it has, in its gill-rakers, a straining apparatus 
scarcely less effective than that of the whalebone whale. To strain out 
the plankton, it holds its enormous but weak-jawed mouth wide open 
as it swims about, permitting the water to flow through its very wide 
gill slits, getting thus not only the smallest animals and plants, but many 
insect larve also of kinds abundant on the open bottom in comparatively 
shallow water. It is, indeed, a living, fine-meshed, water-net. This fish is 
our only proper member of the special class of plankton-eaters, although 
plankton is taken in quantities at times, especially in spring, by a con- 
siderable number of other fishes of various sizes, all with long and fine 
gill-rakers—structures which have, in fact, no other use than to strain 
from the water food particles too small to be taken in any other way. 
The crappies and certain other sunfishes will often so gorge themselves 
with plankton by this means that the bulging of their stuffed stomachs 
can be seen from the outside. Lake herring and white-fish are other 
examples of this class, not dependent, however, upon plankton as their 
most important food. 
While adult plankton-eaters are thus relatively few, it is an interest- 
ing and peculiarly important fact that plankton is almost the sole infant 
food of nearly all our fresh-water fishes, of whatever kind or adult food 
habit. The hatching season of most of our fresh-water species is, in fact, 
the prime season of the year for plankton production in the shallows and 
back-waters where most fishes spawn; and the minute mouths and gill 
slits of the very young are perfectly fitted, without special adaptation, 
for the capture of this microscopic prey. 
