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 i 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, no, 

 in, 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. 



