

Fig. 2. Arbuckle Creek, Florida, choked with water 



hyacinths so that there is little or no oxygen in tne 



water below. (Photo by R. H. Arnett, Jr.) 



lakes the deeper waters may be without oxygen for months at a time. 

 Notwithstanding the severity of the conditions of Ufe in fresh 

 water most of the phyla that began life in the ocean have also be- 

 come established there, but there is usually less variety. Certain 

 rather modern families (Astacidae^ Limnaeidae, etc.), orders, and 

 even classes (Amphibia) are characteristic of fresh water, and do 

 not often occur in the ocean. 



Land plants and animals receive much energy directly from the 

 sun and in the atmosphere have a dependable supply of oxygen, but 

 they are frequently in danger of death by loss of water and must 

 endure extreme fluctuations in temperature. Turbellarians, oligo- 



