The hypothesis of competitive superiority is discussed in the light, first, of the complexity of the phenomena involved in the interaction and distribution of planorbid populations and, secondly, of the relation that is established between the snails of any given region and the infected human or animal host. Special attention is given to the distribution of planorbids in the Amazon, and the endemic problem of the region is dealt with.
Biomphalaria; Residence characteristics; Schistosomiasis mansoni