VEGETATION COMMUNITIES 129 



is left with the greater part of its shallow water of the type which we have 

 described. Vegetation is present from the first in the form of floating 

 microscopic plants, and the dead bodies of these and of the animals 

 present are swept into the depressions and protected situations where the 

 waves do not drag on the bottom. Here vegetation grows in the greatest 

 luxuriance and causes the production of more plant debris, which adds 

 to that already in the protected situations. We then have, after a time, 

 a covering of the bottom by the humus and conditions unfavorable for 

 most bottom animals. The animals of the bare bottom shoals are no 

 longer present in numbers. Small, apparently stunted forms of Lampsilis 

 luteola are found for a time, but are soon driven out by the increase of 

 humus and vegetation. The early vegetation is made up of scattered 

 aquatic plants, such as Myriophyllum and Elodea, and in the shallower 

 water usually bulrushes. 



One of the most distinctive and characteristic forms of such lakes is a 

 transparent true shrimp (Palaemonetes paludosus), about 2 inches long 

 (Fig. 78), which is a close relative of some of the edible marine shrimps. 

 In spring they are found carrying numbers of green eggs attached to the 

 appendages of their abdomens. Another common animal in these 

 situations is the large polyzoan (Pectinatella magnified}. This is a 

 colonial form which reproduces by budding in several directions. It also 

 secretes a clear and transparent jelly. As the number of animals 

 increases the amount of jelly increases on all sides and the animals are 

 arranged on the outside of the more or less spherical mass of jelly; the 

 necessary increase in surface for the growth of the colony is supplied 

 through additional secretion by each new animal added. Some of these 

 masses of jelly reach a size of 6 inches in diameter. They are often 

 attached about a stalk of Myriophyllum as a center. In the autumn 

 they form bodies known as statoblasts (Fig. 77), which are disk-shaped, 

 the center containing living cells and the rim being filled with air-bubbles. 

 The rim of the disk is supplied with hooks which catch onto objects. 

 Probably they must be frozen before they will grow into new colonies 

 for they do so only in the spring. 



Other characteristic animals of this open-water vegetation are 

 shelled protozoa (Fig. 79), water-mites (Fig. 80), and ostracods (Fig. 81). 

 On the stems of the water plants, such as bulrushes and pickerel weed, 

 are the snails (Ancylus) which belong to the lunged group, but are said 

 to take water into the lung and thus do not need to come to the surface 

 for air. Occasional snails, leeches, and midge larvae occur. Water- 

 mites fasten their eggs to the bases of the aquatic plants. Among the 



