PIONEER COMMUNITIES 145 
when he stirs the bottom. Creeping over the plants are the small 
snails (Amnicola limosa) (Fig. 100, p. 146). These respire by means 
of gills. Other snails are also occasionally present. Physa and Lym- 
naea, etc., are always small or juvenile. We have never taken an adult 
specimen of these from the young ponds and in all only a few specimens 
have been taken. These animals get into the ponds that are formed by 
the removal of sand. We are not at all sure but that the few forms 
found in Pond 1 are the result of such entrance, rather than the regular 
establishment of the species. 
Among the bulrushes are a few aquatic insects that belong to the 
vegetation that comes above the surface. One of the most characteristic 
forms is the neuropterous larva (Chauliodes rastricornis) (Figs. 110, 
III, p. 150), which is a marsh form and will drown in water. 
Characters of the association: This association differs from the 
preceding and from the others generally in being distinctly aquatic and 
also essentially independent of the bare bottom and of the surface. The 
animals of this association are, however, strictly dependent upon the 
vegetation for nesting-places, shelter, etc. The mud minnow has been 
studied experimentally and shows avoidance of direct light. 
Tendencies in the association: This association, like all the others, is 
destined not to last; changes are taking place all the time. The chara 
is filling the pond at the rate of one inch a year (58) and is making a fine 
soil for roots of other plants. As soon as the dense chara stage has 
existed for a time we find other plants, such as Myriophyllum, Pota- 
mogeton, and water-lilies. As soon as these have become established we 
have the commencement of the next association. These plants usually 
appear in spots, and in many cases the zones are much less important 
than in the lakes because of the small areas of the plants. We can, 
however, recognize a zone of water-lilies, and zones or patches of other 
plants. 
Just as we noted that the formations of the bare-bottom type existed 
in the small ponds with the Chara, we see also that the surface-reaching 
vegetation occurs with the Chara association and often all three occur 
together. Pond 5 contains a poorly developed phase of all three, the 
bare bottom being of minor importance. Pond 7 contains the chara 
association and the surface-reaching association. Ponds 14 and 30 are 
the best expressions of the surface-reaching type, and Pond 52 is 
the last stage of it. This will be discussed more fully, and we will 
pass directly to the association of the vegetation which reaches the 
surface. 
