THE ENTOMOSTRACA OF EPPING FOREST. 201
parasitic forms, vegetable food is probably in no case altogether
despised, and may sometimes even form the sole nourishment of
certain species. Thus some pelagic forms of Cyclops have been
found living wholly on Diatoms (91), and Prof. S. A. Forbes (93)
mentions a case in which an American Diaptomus (D. shoshone)
was found to be feeding exclusively upon the pollen grains of
the pine which had blown into the water from the neighbouring
forests.
Owing to the very different morphological characters of the
mouth organs, the particular form in which food is taken varies
considerably. But most of the methods of feeding belong to
one or the other of three main types. The Ostracoda, and also
most of the Copepoda, are furnished with more or less powerful
appendages for seizing and tearing their food, and act in conse-
quence as veritable scavengers, by rapidly breaking up and
assimilating all kinds of decomposing animal and vegetable
matter. The Cladocera, on the other hand, have no means,
except in a few cases (e.g., Bythotrephes, Leptodora, &c.), of
disintegrating large food-masses, but depend entirely upon
minute particles suspended in the water. These may consist of
living organisms such as Unicellular-Algae (Desmids, Diatoms,
&c.) and Protozoa (Rhizopods and Infusoria), or of tiny
fragments of the larger animals and plants which have undergone
decay. The third type of feeding, by the imbibition of the juices
of other animals, chiefly of fishes, occurs only in the parasitic
forms, namely, the Branchiura and some Copepoda.
In connection with this subject of the food of the Entomos-
traca, it may be pointed out that the careful compilation of statistics
in regard to the different species would throw a flood of light
upon the interesting problem of the inter-relations of the animals
and plants found in our inland waters.
Economic Importance.
It will be clearly seen from the foregoing paragraph, taken
in conjunction with the fact that the Entomostraca occur in
enormous numbers, that we have to thank these little animals
for disposing of a very large amount of decomposing organic matter
which might otherwise become very unpleasant and even
injurious to health. But these creatures have a further value
for human beings, in that they form directly, or indirectly, a
large proportion of the food of fishes. In this district, to be
sure, the value of fresh-water fishes as food is not very high,