368 
IOWA ACADEMY OF SCIENCE 
they were found in great numbers, almost to the exclusion of 
other forms, in a pool near the mouth of the brook which had be- 
come isolated by the drouth. These worms receive their nourish- 
ment from the ooze, etc., on the bottom. Their food is neces- 
sarily microscopic. 
Mollnsca. — Pond snails, of two species,^ Physa gyrina Say, and 
Limnaea sp?, constituted a great part of the individuals of the 
larger forms in the bottom associations. They were present in 
numbers, crawling about over the bottom and upon the growing 
vegetation as well. It is supposed that vegetable matter composes 
at least the principal part of the food of these snails. The eggs, 
in their masses of jelly, attached to the under surface of the de- 
caying leaves, were found at almost every dredging. 
Entomostracha. — These minute Crustaceans abound in varying 
numbers throughout the whole of the brook, although they be- 
long primarily to the free-swimming association. Of these the 
Ostracoda were by far the most prevalent, the masses of green 
algae being fairly alive with them. A horde of the little ani- 
mals was observed feeding on the soft parts of a dead snail, 
Physa gyrina Say. 
Of the Copepoda, Cyclops sp?, was observed, though not in 
such great numbers as the individuals of the Ostracoda. 
Arachnida. — Spiders of the Drassidae, the ground spiders, 
comprising several undetermined species, form a very important 
part of the transition zone association. They haunt the vegeta- 
tion at the water’s edge and sally forth upon the surface film 
of the water itself to prey upon drowning insects. 
Insecta. — By far the greater number and variety of forms of 
animal life in the brook are those of insects. They dominate to 
a large measure the life in each of the associations. Five orders 
are represented, Ephemerida, Odonata, Hemiptera, Coleoptera, 
and Diptera. 
Ephemerida — May flies. — The nymphs of the May flies abound 
in all of the quiet pools during the weeks of early spring. The 
nymphs of Callihaetis sp ? and Blasturus sp ? were taken in num- 
bers by use of the dredge net on April 10 and April 17. These 
numbers had been very much reduced by April 24, due to the 
emergence of the images. On April 17 the subimagos were 
observed leaving the nymphal skins at the water and taking 
flight immediately. One of these when captured was recognized 
^Determined by Prof. B, Shimek. 
