14 ILLINOIS BIOLOGICAL MONOGRAPHS [208 



stones which make numerous crevices, the shore hne is perfectly straight with 

 never a break or an eddy to disturb the calm. There is movement sufficient 

 to prevent the water from becoming stagnant but not sufficient to cause dis- 

 turbance. The stones are covered with fine algae, thus forming an ideal feed- 

 ing ground. The larger crustaceans, predacious larvae, and even most of 

 the microscopic species find it a poor situation, so that the rhabdocoel enjoys 

 an existence singularly free from enemies, tho on this same account the food 

 supply is limited almost entirely to protozoa. Altho not plentiful in numbers, 

 rhabdocoels are seemingly in control at least along the edge. 



The most nearly ideal conditions for rhabdocoel existence are those found 

 found in permanent ponds. Here the water is comparatively quiet, the plant 

 life abundant and the small animal forms many. The hunting grounds for the 

 small worm are sufficiently rich and the enemies so few that the problems of 

 food supply and protection are almost nil. 



In one such pond eight collections were made at intervals of from three 

 to nine days, with essentially the same conditions and forms appearing each 

 time. This was formerly a httle stream racing thru an open valley protected 

 by low hills on either side. The water is now held back by an earthen dam to 

 make a pool about thirty yards across, with a depth of two and one half feet 

 in the middle. It is kept stirred into a thick muddy semifluid by the wading 

 cattle, and consequently there is little Hfe of any sort. In direct contrast to 

 this, below the dam for four hundred yards the water is clear, kept fresh by a 

 little trickle coming through the spillway, and filled with an abundance of 

 animal and plant life. Early in the summer it spreads out for one hundred 

 yards with a depth of three feet, but later shrinks to one-half this size. 

 The ground around is "mucky," covered thruout themarshy portion with swale; 

 outside there is a region of Carex and Juncus. These, in the first place by their 

 rigidity and harshness serve to keep away disturbing cattle, secondly, act as a 

 windbreak so that not even a ripple disturbs the surface of the pond. The water 

 is more or less filled with masses of Nostoc, Spirogyra, and diatoms. The 

 number of protozoa is very large; then, of larger types, as water beetles, dragon 

 flies, snails, frogs, etc., there is no lack. Members of the genus Stenostoma are 

 abundant everywhere in water from the surface where they swim freely, from 

 deeper more or less muddy portions, and cUnging to water weed which yields 

 great numbers even when transported in a thoroly drained condition. Out 

 in the middle deeper portion are large patches of Nitella, which is nearly 

 covered with organic debris and harbors many specimens. Some half-dozen 

 species were secured, taken from different portions of the pond. 



Another pool where many rhabdocoel species thrived, is in all respects a 

 marked contrast to the above. This is Beatty Lake, a little body of water 

 unique in its surroundings and formation. It is situated at about the level of 

 the Mississippi river, not more than five hundred yards back from the shore 

 line and with the ground between not more than six or eight feet high. 

 The lake is bounded on the side away from the river by a mound seventy-five 



