Fresh-water Snails 
The habits of fresh-water snails are very variable, and each species is 
often associated with a distinctive kind of environment. Some prefer tem- 
porary woodland pools, mudflats, stagnant ponds, large lakes, or swiftly 
flowing streams. Some are amphibious rather than strictly aquatic, being 
found along banks of streams or canals usually a few inches above the water. 
Pomatiopsis (Oncomelania) hupensis of China and our native P, lapidaria 
are such species. Fossaria ollula is almost amphibious but more or less 
confined to shallow marshes. This is also true of F, truncatula, although 
it is also found in other types of semi-amphibious habitats. Pomatiopsis 
(Katayama) nosophora of Japan lives in small ditches or creeks fed by cool, 
clear water. Some snails, that is, Parafossarulus manchouricus, prefer 
the muddy bottom of lakes, canals, and rivers. Pomatiopsis (Katayama) 
gQuadrasi, Melanoides tuberculata, Tarebia granifera, Biomphalaria boissyi, 
and Bulinus truncatus are most often found in rather permanent, slow-run- 
ning waters. Physopsis africana most commonly occurs in quiet shallow 
poois, particularly those having abundant vegetation. 
BIOLOGY OF INTRODUCED SPECIES 
Any snail or slug of no economic importance in its native habitat may be- 
come a serious pest if moved to a new area. One may wonder why such 
animals, when introduced into strange countries, are able not only to be- 
come established, but often to multiply at astonishing rates. 
The answer, of course, lies in the "balance of nature'! or the balanced 
community concept. 
The organization of most communities is extremely complex because of the 
numerous interrelations of the associated species and individuals. The 
plants and animals that are associated to form a community have their lives 
so linked and interwoven that far-reaching changes in the whole community 
may be produced by a change in the relationship between any two species. 
Nevertheless, most natural communities maintain a striking degree of-sta- 
bility. The ability of communities to maintain themselves with only minor 
fluctuations in their composition is due to numerous natural regulatory 
mechanisms, 
These regulatory mechanisms are of two general types, those that reduce 
the number of individuals of the various species and those that assist the 
species in maintaining themselves. Under the latter category is the ability 
of organisms to reproduce at a rate far in excess of the number that will 
actually survive to reproduce again. This high reproductive potential is 
necessary for species to insure continued existence in face of hazardous 
environmental conditions. In doing this they also provide a large margin 
which supports and insures continued existence of their predators and para- 
sites. On the other hand, these predators and parasites, along with other 
