78 COMMUNITIES OF LARGE LAKES 
Other relations are, however, different. As has been stated, there 
are no truly rooted plants in the bottom of Lake Michigan. Those 
attached to the bottom are not rooted in the way that land plants are. 
The things which land plants get from the soil are supplied to the aquatic 
plants by the water itself. The same is true of the bottom animals; 
food is floating in the water in quantities and can accordingly be secured 
without effort, and some animals have the form of plants and simply 
depend upon the food which may be brought within reach by accident. 
Classification of bottom formations: Bottom formations are de- 
termined by depth (and associated phenomena) and bottom. Bottom 
is of greatest importance in shallow water (less than 8 meters). Its 
importance is inversely proportional to depth. 
Within the zone of wave-action conditions are somewhat different 
than below it. Here the kind of animals is determined by (1) strength 
of wave-action, (2) erosion and kind of material eroded, and (3) deposi- 
tion, and animal communities may be classified as those of (1) eroding 
—rocky or stony—shores, (2) depositing or sandy shores, and (3) pro- 
tected situations. 
a) Eroding rocky shore sub-formation (80, 81, 82, 83, 84) (Stations 
ta, 2; Table XV).—There are a considerable number of rock outcrops in 
the bottom inside the 8-meter (26 ft.) line, between Gross Point and the 
mouth of the Calumet River at South Chicago (61). As we shall see 
later, these are of great importance to the animals of the lake. However, 
the communities of such situations are known to us only through the 
study of the very shallow water in the vicinity of Glencoe. Here, attached 
to the rocks by their silk, are caddis-worms (Hydropsyche). (Mr. W. J. 
Saunders has given me specimens of Parnidae (Psephenus) and stone-fly 
nymphs (Perla) taken from Lake Ontario at Kingston, Ontario.) All 
these ordinarily live in swift streams. Under the stones and among 
the algae attached to them are amphipods (Hyalella knickerbockeri) 
and May-fly nymphs (Ephemeridae), but so far as we have been able to 
record these are the only forms common here. The animals avoid the 
waves by creeping under stones or are attached to withstand wave- 
action. The lake trout (Fig. 20) is known to breed on the rocks off 
Lincoln Park. These rocks are then of considerable importance to the 
fish. Some species of small fish may be common here, but they have 
not been studied. 
b) Sandy depositing shore sub-formation, o-8 meters (26 ft.), shifting 
sand bottom (Station 3; Table XII).—On the open shore inside of 1.5 
meters (5 ft.) of water we have found nothing on the bottom. From this 
