in Africa, are endemic to the United States; but our widespread native 
species, Pomatiopsis lapidaria, has been experimentally infected with one 
strain of the human schistosome. Many millions of dollars have been 
spent by the United States and other countries in studying this disease and 
its snail carriers. Extensive research is being carried out on the local 
P, lapidaria. It is extremely important that the vector snails are not ac- 
cidentally introduced into this country. 
Clonorchiasis, caused by the liver fluke Clonorchis sinensis, is a human 
disease especially prevalent in the Orient because of their practice of eat- 
ing uncooked, or partly cooked, fish. The snail mainly responsible is the 
operculate Parafossarulus manchouricus, Similar liver flukes, Fasciola 
hepatica and F, gigantica, are serious parasites of sheep and cattle. These 
parasites are carried by such snails as Fossaria truncatula in Europe and 
parts of Africa, F, ollula in Hawaii, and similar lymnaeids in other parts 
of the world. 


Paragonimiasis, or the lung fluke disease, is less damaging to the body 
than bilharziasis and clonorchiasis, but nevertheless may be fatal. The 
adult worms measure about one-half inch in length and live on the inner 
walls of the lungs, where they produce tuberculosis-like lesions. It has 
been estimated that there are about three million cases of paragonimiasis 
in Asia and West Africa. The mainvector snails of Paragonimus are 
Semisulcospira libertina and Tarebia granifera. The latter species has 
been accidentally introduced into Florida and has subsequently become 
established in the State. 
It may seem that introduced snails and slugs would be of little concern to 
man if they could be kept out of gardens, orchards, and greenhouses, and 
are not capable of carrying disease organisms. However, this is often not 
the case, although the undesirable effect of snails in uncultivated areas may 
not become apparent immediately or even for considerable time. 
Effects of Natural Communities of Plants and Animals 
Natural communities, if not disturbed too greatly by outside influences, be- 
come stabilized structural entities, consisting of all the plants and animals 
which normally constitute the community coexisting in a more or less mu- 
tually satisfactory association. These animals and plants stand in a spe- 
cial relation to one another, the community organization depending chiefly 
on the manner in which they are interrelated, both among themselves and 
with their physical and chemical environment. Plants not only provide 
shelter and protection for other plants and animals, but supply food for the 
vegetarians or herbivores, which in turn tend to keep the plants from be- 
coming too abundant. The number of plant feeders in the community is to a 
large extent controlled by carnivores and parasites, which in turn are regu- 
lated by other carnivores and parasites. 
