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THE ESSEX NATURALIST.
important perhaps is the elucidation of the so-called "food
chains" which exist among the inhabitants of any given piece
of water. Everyone knows of course that all animals are ulti-
mately dependent upon plants for their food, but that very often
the dependence is not direct, but through one or more inter-
mediate animal links. In water, this chain-like relationship
between the organisms is very well illustrated. Comparatively
few aquatic animals derive their food directly from the higher
plants. Most of them depend in the long run upon the micro-
scopic plants—especially the algae. These form the immediate
food of a variety of animals, sometimes even of larger animals
such as molluscs, but usually of the smaller forms such as Pro-
tozoa, Rotifers and Entomostraca ; these are eaten by such
creatures as insect larvae and young or small fishes, and these
again by larger fishes. Fish may be eaten by certain mammals,
both aquatic and terrestrial (e.g. otter, man, etc.), or by birds,
which in turn may fall victims to certain mammals, including
man. The foregoing is necessarily but a rough illustration in
general terms of the idea of the food-chain. In reality the facts
are usually more specific and often complicated in a variety of
ways. It is one of the tasks of the aquatic biologist to accumu-
late as many facts as possible bearing on this question ; what
is particularly wanted is the detailed working out of the food-
chains that exist in well-defined areas or single pieces of water.
The value of such knowledge in connection with practical fishery
questions would be very great.
Another of the problems of the biological environment is
indicated by the word "parasitism" and yet another by
"symbiosis." Both of these refer to aspects of organic relation-
ship which are as well represented among aquatic as among
land organisms, and the study of these particular relationships
among aquatic organisms may actually lead to results not to be
obtained in any other way. This is owing to the fact that many
of the smaller aquatic organisms are sufficiently transparent to
allow of the relation existing between the host and the parasite
in the one case, and the symbionts in the other, being observed
in the most intimate way while the organisms are alive.
Further problems of the biological environment of a most
interesting character are raised by the relation existing between
the organisms in the water and those on the surrounding land.