30 ILUNOIS BIOLOGICAL MONOGRAPHS [224 



with any season or with environmental conditions, since it is a constant pro- 

 cess, but that the rapidity of the growth of zooids, i.e., the number of genera- 

 tions developed, is dependent upon these surroundings. 



Stenostoma tenuicauda von Graff 1911 

 Most often in ponds where Stenostoma leucops is present, in larger or smaller 

 numbers, there may be also several different species of rhabdocoels, but gen- 

 erally no other members of this genus are to be found. In several of the ponds 

 at Fairport, Iowa, however, Stenostoma tenuicauda was also present and was in 

 this instance the more frequent with very nearly the same life-habits and the 

 same environment. The two species existed side by side, but there was a great 

 difference in their rapidity of movement. Stenostoma leucops is much more 

 regular and quiet, holds more constantly to its course, while Stenostoma tenui- 

 cauda shows a tendency toward a greater amount of action. In appearance, 

 too, they differ decidedly; while the former is slightly opaque, the latter is quite 

 clear and transparent, slightly yellowish in tinge. The slender tail region 

 is also conspicuous when taken together with the blunt and heavier condition 

 in Stenostoma leucops. 



Stenostoma giganteum nov. spec. 

 Figures 46 and 48 



This species was collected in numbers in a clear pond with sandy bottom 

 near Grandview, la. The water was very warm at the surface and cold a few 

 inches down. The drainage area was small, with no direct inlet, so that under 

 all ordinary conditions very little outside water entered and since most of 

 the water was seepage, there was no chance for any number of species to be 

 washed in. A small rivulet carried off some surplus, but for the most part 

 all current was lacking and in fact not even a disturbance of wind could be 

 detected. The conditions were very nearly those of a balanced aquarium. 

 The comparative shallowness, with sandy surrounding area prevented any 

 silt from being deposited. Then, too, the amount of disintegrating organic 

 matter was minimum. The plant life was profuse, enough to take care of all 

 the carbon dioxide generated, so that the water was always clear and fresh. 

 The animal life was composed of a few fish, mostly Amia, a few stray turtles, 

 microscopic forms like small Crustacea, protozoa, and many rhabdocoels. The 

 lack of disturbance or change and the constancy of inter-relationships of various 

 species precluded the possibiUty of great differences in the general fauna and 

 flora and brought about persistence of the same t)^es and a continuation of 

 the same relative conditions. The situation in the present instance was, 

 perhaps, slightly different from that of other years, since by a break in the 

 shore line the river had swept down thru the pond carrying away the whole 

 thing. As a result all the pond Hfe was new, at least only that left when the 

 water receded. The rhabdocoels were floated out from masses of Chara pulled 

 up from the bottom where the water was eighteen inches deep, but they were 



