SUCCESSION OF COMMUNITIES ia3 
formations are quite similar in many respects to the formations of the 
vegetation in sluggish rivers but resist lack of oxygen and stagnant 
water much better. 7 
d) The anaerobic formation.—This is the bottom and deep-water 
formation. We have already stated that the circulation of water (see 
Fig. 10, p. 61) is not known for any of the lakes discussed. Old lakes like 
those about Chicago are usually covered with humus on the bottom. In 
this humus and probably just above it there is little or no oxygen. 
Analyses of the bottom water from ponds with humus-covered bottoms 
showed that it contained no oxygen. The open water of the lakes with 
the incomplete circulation in summer is without sufficient oxygen to 
support life, below the level of circulation (Fig. 11, p. 61). There 
are, however, numbers of animals that pass the summer under these 
conditions (110, t11). These are protozoa belonging to eleven genera, 
worms belonging to two genera, one rotifer, one ostracod, and the small 
bivalve (Pisidium idahoense). Dr. Juday kept these animals in jars 
without oxygen and observed their activities. The rotifer was always 
active. The ostracod showed little activity, and the bivalve kept its 
valve closed, showing no activity whatever. 
There are occasional midge larvae in the mud of such bottoms, but 
they are rare. Some of these have haemaglobin in their blood and are 
supposed to be able to use oxygen when it is present in the minutest 
quantities. In the open oxygenless water there are phantom larvae 
(Corethra) which are able to carry a supply of oxygen with them from 
the surface. 
III. SuccrEssion In LAKES 
The general tendency of succession in lakes has been indicated. The 
first formation is the bare-bottom type, which is locally transformed to 
the vegetation of open-water type. This usually begins in the protected 
situations first; the bays are ecologically oldest. These bays pass 
rapidly from the third open-lake type to the bay conditions. When such 
a stage has been reached the situations that have a less degree of protec- 
tion from waves have reached the second stage and we have lakes as we 
find most of the larger ones about Chicago. They contain, at. various 
points, the three formations which we have discussed. The lake is 
reduced in size by filling near its shores and the lowering of its outlet. 
The older stages are continuously encroaching on the younger. The 
area of barren shoal is constantly becoming less as the lake fills and the 
outlet, if it has one, is lowered. Around the shores the development of 
prairie or forest is usually well begun and one or the other of these types 
of land vegetation finally displaces the lake. 
