In the scale of animal life the vertebrates or back-boned animals
succeed the insects. Beginning with the fishes, we find that in late
autumn they mostly seek some deep pool in pond or stream at the bottom
of which the water does not freeze. Here the herbivorous forms eke out
a precarious existence by feeding upon the innumerable diatoms and
other small plants which are always to be found in water, while the
carnivorous prey upon the herbivorous, and so maintain the struggle
for existence. The moving to these deeper channels and pools in autumn
and the scattering in the spring of the assembly which has gathered
there constitute the so-called "migration of fishes," which is far
from being so extensive and methodical as that practiced by the
migratory birds.

Many of the smaller species of fishes, upon leaving these winter
resorts, ascend small, clear brooks in large numbers for the purpose
of depositing their eggs; as, when hatched in such a place, the young
will be comparatively free from the attacks of the larger carnivorous
forms. Among the lowest vertebrate often found in numbers in early
spring in these meadow rills and brooks is the lamprey, _Ammocœtes
branchialis_ (L.), or "lamper eel," as it is sometimes called. It has
a slender eel-like body, of a uniform leaden or blackish color, and
with seven purse-shaped gill openings on each side. The mouth is
fitted for sucking rather than biting, and with it they attach
themselves to the bodies of fishes and feed on their flesh, which they
scrape off with their rasp-like teeth. Later in the season they
disappear from these smaller streams, probably returning in midsummer
to deeper water. Thoreau, who studied their habits closely, says of
them: "They are rarely seen on their way down stream, and it is
thought by fishermen that they never return, but waste away and die,
clinging to rocks and stumps of trees for an indefinite period; a