209] TURBELLARIA FROM THE MISSISSIPPI BASIN— HIGLEY 15 



feet in height which bends around to the southward, forming a nearly perpen- 

 dicular wall. Toward the north, the sand slopes away gradually. Surrounded 

 on all sides by barren sand, the pond is essentially a 'hole' with the bottom so 

 near the river level that the water never seeps away, and with enough draining 

 in to prevent its drying up completely. On the west and south the wind is 

 entirely shut off, while even on the north and east the ground is high enough 

 to prevent much of a ripple ever disturbing the surface from that direction. 

 There is almost no vegetation within a hundred yards of the pond in any direc- 

 tion. This is partly due to the blowing of the sand which covers everything. 

 In one place bush-Uke branches can be seen, all that remains unburied of three 

 tall locust trees. The pond bottom is sandy, very solid, and smooth thruout. 

 The banks slope very gradually into the water, which is quite clear, from two 

 to five feet deep, very warm at the surface and cold two feet down. Tho used 

 by cattle to some extent for drinking* the sand, when stirred up, quickly settles, 

 leaving the water as clear as ever. There are no rushes, or other plants of this 

 type, which reach up above the surface of the pond, so that the broad expanse 

 is uninterrupted. Near the shallow portions within the pond, are great masses 

 of very coarse Spirogyra, showing a brilliant green against the sandy back- 

 ground. There are also heavy and dense patches of Nitella, sheltering at least 

 five species of rhabdocoels, all active and well-developed specimens. In the 

 deeper portions are large clumps of water lilies and several other flowering 

 plants, such as arrowhead, and floating around at the surface, sheets of Clado- 

 phora and other algae. Thus, while the plant life is very profuse, the animal 

 forms are much fewer, and larger types seem lacking. One notes the absence 

 of such ordinary types as frogs, clams, snails, waterbeetles, and dragonflies. 

 Out on the banks a sand toad is common, together with four species of turtles 

 and a Httle sand lizard. Altogether, the conditions prevaihng seem to be 

 ideal for several species of rhabdocoels tho not so favorable for most others. 



A rather surprising situation was that encountered in an old boat anchored 

 high and dry in a httle clump of willows. It was protected from the wind 

 and open to the sun so that evaporation was rapid. However, the two or three 

 tubfuls of water which the boat contained must have been placed there two 

 months before, at the time of high water, as there had been no rain. The whole 

 boat was nearly filled with three species of Spirogyra all conjugating. It was very 

 luxuriant and harbored copepods, ostracods, diatoms, rotifers, and lower algae. 

 There were no insect larvae, as might be expected, but many species of proto- 

 zoa and such simple organisms. Rhabdocoels were there in abundance, the 

 ever present Stenostoma and two other species, all thriving in their prison, a 

 little world entirely isolated, but perfect. 



Thirty yards away from the old boat was a muddy pond which well illus- 

 trates a different type of condition. Early in June, the overflow flood-water 

 spread out over a forty acre corn field; much of it drained away and by the 

 middle of July, only a depth of a few inches was left. For a rod back from the 

 actual water edge the ground was extremely mucky and soft down to a depth 



